literally all of them are my favorites lmao

For Starcraft definitely play the original + Brood War. Note it is kind of an outdated interface, personally it doesn't bug me at all but that's because I burned hours of my life on this game and I'm a strategy game nerd to begin with. Anyways the gameplay is top-notch. IMO this is the greatest RTS game ever made. Anyways then move on to SC2 and the expansions if you liked the original.

For Warcraft all the current lore is in the MMO (World of Warcraft) but I know that's not everyone's thing. Warcraft 3 + The Frozen Throne is accessible and easy to play and has some super fantastic missions and stories and characters in it. Bonus points if you go back and play Warcraft 2 but its interface is even more primitive than StarCraft's so just kinda be aware.

For Diablo you can pretty much jump into Diablo 3, with the expansion and everything it's a solid all around game for both beginners and veterans to the genre. If you're feeling adventurous give Diablo 2 a shot.

Heroes of the Storm - it's a MOBA, it's okay, it's fun for a few days if you're already a fan of Blizzard franchises aka Blizzard Trash (like myself)

Hearthstone - it's a card game, it's not really my thing (I prefer other card games) but a lot of people seem to love it. It's based off of the Warcraft universe so you'll get more mileage out of it if you're familiar with Warcraft.

Overwatch - it's not out yet, it's gonna be a shooter (like TF2)

The only really "lesser known" Blizz games I can think of off the top of my head are SNES games like The Lost Vikings and Rock n' Roll Racing. These are actually downloadable from battle.net for free

I am literal Blizzard Trash so yeah basically ask me if you have any more questions