flip phones, imo, are too much work to answer the phone. I have to fish it out of my pocket, look at the front screen, decide if i want to talk to them, and THEN open the flip to answer. PITA, if you ask me.
candybars are the best thus far, plus, as has been noted before, the less moving parts the better.
slider, i wasnt sure i'd like it, but i loved my n80, which is a slider. the smaller form of the flip, with the convenience of a candybar.
i don't do the swing-around swivel bs, though.