My preference on bras is: as thin, smooth, and seamless as possible. I hate seeing girls with fancy lace bras or massively padded bras that end up having lumpy, weirdly-shaped boobs under their clothes.
The more of the girl's natural shape that I can see, the happier I am. I don't care if they're big or small.
I like the push up bras, just to add to the discussion, you should also consider balconette bras? these are just fantastic. As to the material and colour there are types for all occasions and each very sexy in their own right.
I depends on the situation, to be honest. I have a relatively small chest so pushup bras look the best on me for special occasions, but bras with lots of coverage feel the most comfortable in my opinion.