Yeah, Blizzard is definitely up there among the most successful.
For me they're mostly great because of their legacy and not for their current output (last 10 years).
Historically Valve, Sega, Rockstar North, Capcom and Game Freak are imo some of the best in the industry as well but personally I wouldn't rate them that high when speaking current top developers.
Besides, who's the "best" developer in the industry is kinda subjective.
I've never been into MMOs or TCGs and regarding RTS I was always more of a Westwood fan (Dune, Red Alert, Tiberian Sun, etc).
Though Diablo 2 is my favorite ARPG/hack and slash ever so they actually catered more to my tastes in the early 2000s where I think they had more success in creating great atmospheres in their games. Nowadays, gameplay is still rock solid (Diablo 3, Overwatch, Hearthstone, etc) but I don't think the atmosphere is as rich.
The current (last 10 years) top 15 developers for me:
1. Bethesda Game Studios
2. Naughty Dog (might be number 1 again with TLOU2 but UC4 disappointed me a bit)
3. Nintendo EAD
4. Ubisoft Montreal
5. FromSoftware
6. Firaxis Games
7. Blizzard North
8. Rockstar North
9. Square Enix
10. CD Projekt RED
11. Gearbox Software
12. Bioware
13. PlatinumGames
14. Crystal Dynamics
15. Bungie
