q1: best diplomacy ai prize goes to: Civ 5, by far. Why, because it really is diplomacy, not just stalling until i can backstab you.
q2: depends on my mood. on average ~500 turns.
q3: space empires 4 for HUGE diversity. There is nothing you can think of and not be able to do, although not always useful.
sword of the stars for the tactical combat. its simply beautiful.
MoO2 puts all the elements best together overall.
Now, on a sidenote: i played quite a lot Gal Civ 2, and i am a founder for Gal Civ 3, but 2 has been a DISAPPOINTMENT.
Why: 1 interface is weak. far too many clicks for simple things
2 ship builder sucks. for those of us who are not professional artists is just too time consuming to get anything of use done.
3 colony management: that SLIDER really sucks. Its pointless.
4 game manual lacking badly in anything resembling something useful
5 trade routes, dont even get me started
6 population happiness/growth, was it ever considered ?
7 AI, i suppose it comes from Artificial Ineptitude
8 the last 2 expansions really felt unpolished and unfinished.
9 and i can go on and on. BUT what REALLY SUCKS: that game has NO TACTICAL combat. Just some bs replay of some precalculated events. You build ships, send them in combat and watch the replay. Oh come on..... just look at the list at q3.
PS: are you guys serious? for a 4x game you advertise about the ship builder????? Instead of working on diplomatic consistency, strategic depth and tactical prowess?