i cant say just one, because my skin has been through different stages of acne-covered,oily,dry/flaky,reddened,splotchy,sun-spot covered..ugh, you name it.
but thinking about this question, over the years the foundations that i've really
really had good results for that time being:
revlon age defying (when dry and wrinkly)
lancome teint ideole (when semi oily and semi flaky)
revlon colorstay (always come back to this to KNOW i wont aggravate acne)
estee lauder ideal matte...or double matte (when really oily)
most recently i got the Dior forever extreme wear, and while i like it (it goes on
velvety smooth), i feel like i have to touch up oily areas (my nose) too much
through the day....so maybe it will work better when my skin dries back
up this winter