I voted 75%. I'd prefer to say I prefer to win 51-74% of the time, but the reality is that I like to win a little bit more than that, so 75% is the most realistic choice.
Basically, if it's never even close, the mid-to-late game isn't interesting. If I'm playing Deity, I'm almost always going to get steamrolled, and that's no fun (though I was quite pleased the two times I won a Civ III war on Deity - just for another AI to steamroll me). My ideal is that I can usually pull off a victory, but with it being close at the end, and with a chance of a loss if I misjudge something. In practice, it usually isn't that close at the end, but as I have at least a few difficulty struggles, I tend to be happy.
A couple games that I'd say were very close to the ideal:
[*]A Civ4 World Map Better AI game where I played as England, was the most advanced, and went for space race. The AI actively attempted to prevent me from winning, which was enough to make it very difficult and close, so much so that I nuked myself just to stop their Stack of Doom. However, the AI actually became too savage in attacking me, such that it became unfun fighting nearly everyone with no real hope of peace, and I didn't finish that game.
[*]The current Civ3 COTM. For details you'll have to see the spoiler threads, but it was set up as very much an uphill battle, and that resulted in a very fun situation where nearly everything was in the AI's favor, but with some cunning tactics I was able to create at least a chance of - though by no means a guarantee of - victory.
The other factor that causes me to choose 75% is that in Civ3 I usually play Monarch, not Emperor. I can win on Emperor, but less than half the time. On Monarch, it's rare that I lose. Thus, most of the time, I like to have a pretty good chance of winning, but every so often I'll play a game where I'm not likely to win. Civ4 I'm not as good at, so I usually play noble, where I usually win, but by less of a margin than Civ3 Monarch.