Gen 3's legendaries are the best. Gen 1 and 2 are okay, birds are pretty mediocre but Mew and Mewtwo make up for it. Gen 2 is good, beasts plus Lugia and Ho-oh are pretty good. Weather trio is awesome, not abominations like Palkia, Dialga and Zekrom (Reshiram is okay). I love Lati@s, Regis are okay and Deoxys/Jirachi are pretty good as well. Gens 4 and 5 are kind of overkill in terms of the number of legendaries, so Gen 6 only having six of them is alright not mentioning the fact that they're all pretty good. Overall, I'd say:
1. Gen 3
2. Gen 2
3. Gen 6
4. Gen 1
5. Gen 4
6. Gen 5
