It's real tough to isolate 3 themes out of the general awesomeness of LEGO's
Anyway here's mine...
Best:
-Pirates - my all time favourite
-Western - not many sets but they were all really really good, really underrated I think.
-Castle - I wasn't much into it but my girlfriend and the new Castle Town Square made me change my mind, and IMO the most beautiful MOCs are made around that theme.
Worst:
-Bionicle - doesn't look much like lego
-Sport - uh well...
-Recent years Space - they just look weird to me
I do agree with some posters about Star Wars... I find many of the sets beautiful, but hardly creative nor playable, mostly just good to sit on a shelf and that's not quite the idea I have of Lego's. Kind of a love/hate thing
Re Technics, I really loved the old sets which were build for the Technics "mini" figures, these packed a lot of fun for little boys (and girls) who love playing with huge badass cars/planes/submarines and whatnot
And City is in a league of its own, it's kind of the backbone of LEGO.