#1 was so good because of the planets/atmosphere and Saren, the best bad guy theyve had in the series so far. Without him, it suffered a bit. #2 final boss was a joke. It didnt even have a name. "Big Reaper", wtf. whatever. Sorry for spoilers but it doesnt even matter. Beat the other ones injate. #3 is pretty good.
But Andromeda will kill them all. #1 is still the best currently.