my vote goes to Aardman animations, due to the sheer effort they put into it- Nick Park's a genius and there's so many little visual gags and references in their work you always come back, just to check you haven't missed anything!
I 'd take pixar over dreamworks any day of the week though- dreamworks just don't seem to have good characters and they always appear to be copying others, while pixar films are consistently good.
also, you missed out universal pictures; they did ice age and despicable me,
With just their first strike against them (although I don't fully say that since I have yet to see Cars 2), I still say Pixar is the most reliable as of the last 20 years. If we're talking all-time, then Disney has so many classics that it comes in first, but Pixar films have always been more appealing to me. To their credit, I'll say Dreamworks has improved in the past few years (Kung Fu Panda, Despicable Me), AND Disney is attempting a revival with "Princess..." and Tangled, so the animation field is again a hotbed of entertainment! Whatta choice!