Ok a few cavets:
1. Only core games are included, not expansion packs.
2. Only games from Blizzard three core franchises are included, starting with WC2.
Obviously it's very difficult to compare a game that came out in the mid or late 90's to one coming out today, but I'm talking about in terms of quality of game FOR THE TIME THAT GAME came out.
Also note, I'm not asking what your least favorite game is, I'm asking what yu think their worst game is. An example would be, if you personally hate the RTS genre or you personally hate the MMO genre, that doesn't automtically make an RTS or MMO their "worst" game, try to judge a game faily based on the quality of the game for it's genre. It's certainly possible that the game you think is the worst is also your least favorite, but I just wanted to draw the distinction.
Also note, worst does not necessarily mean bad. In the Olympics in the 100m dash someone will come in last and they will have done the "worst" for that heat, but that doesn't make them slow.