Jaina. The Ranged Assassin new accounts can pick as their first Hero. She's also given for free if you bought the Spoils of War edition of Warcraft III: Reforged. Often described as simple to learn but difficult to master, Jaina is capable of incredible burst damage, impeding escapes of any kind, shredding Minion waves in moments, clearing camps quickly and safely... but is frail and reliant on getting in close to get best results, requiring lots of experience and good judgment to make getting in close something that secures a kill for her team, rather than getting her killed.
Raynor. If you get into Heroes of the Storm via Recruit A Friend, you get Raynor, and the devs intended him to be an easy Hero for beginners to get into, so he's early. In practice I'd recommend new players stay away: good Raynor play puts a lot of emphasis on stutter-stepping and having good judgment about where to apply his versatile capabilities, which are qualities that favor veteran players a lot more, and there are many Heroes who are much more genuinely friendly to a learning player.
Sylvanas. Like Raynor, being recruited by a friend gives you Sylvanas for free. Unlike Raynor, your account has to reach Level 10 for her to be unlocked, so I'm covering her after Raynor. A Ranged Assassin who can enable safe pushes on Structures at any stage of the game. Like Raynor, she's probably imagined as newbie-friendly, and I'm skeptical due to how important stutter-stepping is for her and how she has a lot of impressive tricks that require good familiarity with the game to put them to good use instead of getting her killed.
Tracer. If you bought Overwatch: Origins Edition, you get Tracer for free, so she's the first Overwatch Hero I'm covering. A highly mobile close-quarters Ranged Assassin, able to dive deep into the enemy lines, delete a vulnerable target, and zap back to safety. Also able to pursue a fleeing target and keep applying damage without falling behind, securing kills on enemies who might otherwise escape successfully. Also actually quite good at taking Mercenary Camps by herself, all the way up to Bosses if you can stay on top of the micromanagement needed to properly kite them.
Li-Ming. One of the Heroes hit worst by the game's issues with explaining itself clearly. Reliant to an unusual extent on projectiles that can not only miss the desired target but can in fact be blocked by an undesired target, exacerbated by her default pressures to fight at long range, but also with very high damage output and a low Mana drain rate letting her keep up the pressure for a long time.
Valla. A mobile, versatile, deadly Ranged Assassin, hampered primarily by poor HP. She can do basically anything you might want a Ranged Assassin to do, just so long as you minimize enemy opportunities to get damage on her.
Nova. One of the game's four 'Stealth Heroes', as classed by internal game metrics. When played well, appears from the shadows without warning, finishing off weakened enemies who thought they were safe, bursting down unsupported frail Heroes, and otherwise securing kills. When played poorly, gives the enemy team lots of free progress on Quests tied to hitting Heroes.
Sergeant Hammer. The only Hero in the game who can just sit in front of Structures and blast them from safety forever. As such, one of the strongest split-pushers, demanding a response if you don't want her demolishing your Structures at no real cost.
Zagara. A Hero who can readily demolish unprotected Structures if ignored, and tends to win duels if her opponent is unwary. Can also Talent into being very deadly to Heroes, though she can struggle against Heroes with a lot of area firepower as they can just quickly delete the summons that are where most of her kit is tied up.
Cho. The physical aspect of Cho'Gall's unique two-in-one design. More mobile than you might expect, able to survive a lot of punishment, and with decent damage in his own right.
Gall. The blaster caster portion of Cho'Gall's two-in-one design. Damage output that's almost impossible to suppress, and which doesn't disrupt Cho's activities, making the pair one of the more relentlessly deadly pursuers in the game. Also gives them a bunch of truly unique advantages, and, well, some unique disadvantages. I'd also argue Gall is one of the better Heroes to potentially try to ease a newbie into the game with, since Gall doesn't have to concern himself so much with positioning and map awareness and all, and in fact is liable to practice map awareness just from getting bored during travel and whatnot.
Tassadar. A Ranged Assassin with incredible area damage output and the ability to cut off escapes and pursuits.
Tychus. A short-ranged gunner who has no respect for your high max HP, with surprisingly good area damage.
Nazeebo. A Ranged Assassin whose max HP can get quite high if he's allowed to harvest enemy Minions, and can push a lane quite effectively if left unopposed. Can be pretty deadly to Heroes too, but everything he does is difficult to land on an unpredictable moving target.
Zul'jin. A Ranged Assassin who keeps up the pressure and punishes careless positioning, in part by steadily growing stronger the more he gets to hit Heroic enemies. Surprisingly Mana-hungry given how he's focused on Basic Attacks, especially if he's careless with his HP.
Fenix. A Ranged Assassin heavily reliant on his strong Basic Attacks, who can trivially shrug off modest punishment if given a few moments peace between fights.
Kael'thas. A Ranged Assassin who is devastating to teams that like to clump, with excellent waveclear and good ability to peel diving enemies off himself.
Kel'thuzad. A combo-focused Ranged Assassin who punishes teams that clump, move little, or move predictably, who can output extreme damage in the early game if his combos are mastered and is fully capable of setting up for teammates.
Azmodan. The highest-HP Ranged Assassin in the game, with strong ability to clear waves and push lanes (And do so without necessarily even entering a lane), the ability to pick off low-HP targets from long range, and surprising capacity to simply murder foes who try to dive him.
Cassia. A Ranged Assassin who has incredible ability to absorb damage and dish it back out, but reliant on continuous movement to stay alive, and with a shorter range of effective engagement than most Ranged Assassins. Also a great option for training yourself to stutter-step habitually.
Mephisto. A Ranged Assassin who excels in big team fights where he can constantly reuse his Abilities because he hit so many enemy Heroes. Mostly provides damage, damage, and more damage, but can often do so very reliably and safely, including that one of his Heroics lets him attack the entire enemy team no matter their positions, potentially unstoppably executing someone who thought they'd gotten away.
Probius. A frail Ranged Assassin with absurd damage output who can stay in the field indefinitely so long as his Pylons aren't being destroyed too often and he doesn't let enemies hit him much.
Orphea. A close-quarters Ranged Assassin with shocking burst damage, excellent area damage, and great sustain damage if she keeps landing Shadow Waltzes. Particularly good at dueling large Heroes. Not so good at hitting Mercenary Camps.
Gul'dan. An incredible damage dealer who tears through his Mana absurdly quickly, having to burn his own HP if he wants to keep his expensive damage flowing. He can get that HP from his enemies, but he also benefits from a Healer in the area, especially a Healer whose healing is duplicated across allies so his self-harm isn't actively pulling healing away from his allies.
Falstad. A highly mobile, slippery Ranged Assassin, with excellent pursuit and escape ability, high damage output, and more ability to absorb punishment than you might expect.
Lunara. A Ranged Assassin who excels at kiting, chasing, and keeping out of reach of Heroes who lack burst mobility and immobilization tools, with some late-game utility not usually found on Ranged Assassins, such as the ability to clear disables from an ally. Also really good at semi-passively monitoring nearby locations.
Junkrat. A Ranged Assassin with probably the best 'poke' in the game, able to very safely harry enemies from a distance for essentially forever if no one comes to stop him, which he has the tools to punish and/or escape such attempts. Technically has no HP sustain, but his most popular Heroic allows him to circumvent this issue and indeed to make very aggressive pushes deep into enemy territory and then easily escape retribution.
Genji. An in-and-out close-quarters Ranged Assassin who readily picks off enemy Heroes who end up low on HP anywhere nearby him, with excellent ability to shrug off significant punishment if coupled with a good sense of timing. His Mana pool runs dry fast and he has no self-healing whatsoever, though.
Hanzo. An extremely lethal Ranged Assassin who can strike at odd angles, becomes better at pursuit and escape when rough terrain is about, and once Heroics are online can support allies in entire other lanes. Has low HP, no HP self-sustain, and can be pretty Mana-hungry.
Chromie. A Ranged Assassin with exceptional ability to harry enemies from a safe distance, and exceptional ability to single out a target for death. Also has a minor gimmick of picking up Talents earlier than normal, which can particularly be significant when Heroics are almost online -and she already has hers.
Greymane. A hybrid Ranged Assassin/Melee Assassin who has excellent damage output no matter what he's doing and no matter what he's fighting. Has no self-healing and offers no utility beyond his incredible damage, though.