Honestly, I'd say B'Day. Bey is known for giving us that big band sound, and B'Day is full of it. This was Bey at her finest until 4 arrived. B'Day. Bey's day? Yeah, that's currently her signature album. It's way too early to say anything about 4.
No shade to Beyonce, but I honestly do not believe she has a signature album yet. There's nothing wrong with that, especially since we've yet to see the best of Beyonce. But I really don't think she has one at this point. 4 has the potential to be that album, but as time goes on, the chances of it becoming that album for her are becoming slimmer and slimmer.
Anyway, if I had to vote for one, I would say I Am... Sasha Fierce.
Graduation will be his most remembered album. I still remember how him and 50 had their albums coming out the same day (Sept. 11. 2007) and how he beat him to #1
MBDTF could have been his signature album if he actually did some promo
I personally think College Dropout should be remembered as an wonderful debut too. Mind you I love all of his albums so it was hard to choose.