Everyone is different with their heroes and what they like. Some like heroes based on play style, some based on aesthetic, some on attachment from their original game. You may have tons of fun playing Malfurion and Johanna, for example. A lot of folks I've played with find those two heroes to be among the blandest of their specific archetypes, however.
If you're looking for the most straightforward three to check out the game for the three main archetypes, they are Lili (support), Valla (assassin), and Muradin (warrior).
Play some AI and level up whoever you want. You can change the difficulty on the left hand side. Even on the hardest AI levels it will become a cakewalk very quickly, though, so once you're comfortable wade into Quick Match with real people if you'd like.
To compare it to Vainglory (which I also play), Valla is kind of like the Koshka of HOTS as far as her place in the balance of the game and ease to pick up. Johanna is not unlike Catherine with a dash of Ardan. Very stun and survival centric. Lili is like a much, much easier Adagio (in some respects) and in general HOTS support are more "heal-centric" than Vainglory support. HOTS support are a lot like MMO support honestly.
But don't do this until you've learned the game a bit and how it is supposed to be played or you'll have a bad time.
Murky is arguably the most fun hero in the game, but he's also both (A) not easy to play well without excessive feeding (dying) and (B) completely unlike anything else in the game due to his death mechanic so he's often hard for new players (and old ones) to understand when being played or having him on their team.
You should definitely give him a look if you stick with the game and enjoy it, however. Plus all of his skins are amazing.
