Midline for AI, high-tech for me (sometimes). Midline offers a lot of flexibility in tactics and loadouts, is decently fast while having ballistic weapons, and has a decent offensive focus. High-tech most often has better playerbait ships, though.
By ship type, high-tech rules the frigate wolfpacks (no surprise, seeing how frigates fit high-tech doctrine well), but destroyers and cruisers belong to the midline school of design. I don't typically get the capital ships I want, not before I "win" the game anyway, so I don't have a set preference. Legion, Conquest and Paragon are all good.