I love Three, but AWDF, One Touch & TIMW are all fab too.
I choose Three because it has some beautiful hidden gems on it (Situation's Heavy, Conversation's Over, Whatever Makes You Happy and Maya) that are up there with Sugababes best known songs in terms of quality. Plus it has what is, in my opinion the best Sugababes song. Too Lost In You. Well, joint best with Freak Like Me. Plus it was their last album to have that slightly indie & alternative sound about it (Singles aside), from TIMW it was a downward spiral of commercial pop that just got progressively desperate.
The 'Three' era also had some great b-sides that are well worth checking out!
I think I should give Change a special mention too. Purely because it has three of my favourite babes songs (Denial, 3 Spoons of Sugar, My Love Is Pink)
I can't stand either C&S or Sweet 7. Limp is the only word that springs to mind. Don't even get me started on Freedom.