It depends on my mood. I am expantionist by nature, so I tend to like large map with many cities (perfectionist ICS approach - meaning most cities work close to 21 squares.
On small maps, OCC is really starting to grow on me. I'm even starting to look competitive, although OCC #16 will be the real test, since I've been working backwards starting at #15 (and oddly enough, #15 was easily my worst showing

)
I also enjoy some of the scenarios, as long as they aren't total war. Kind of makes me glad I don't have MGE

For some reason, most of the scenarios I've enjoyed most seem to be on medium maps. So, based on that:
Small map: OCC
medium map: Scenarios providing a balance between war and peace
large map: perfectionist ICS
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April Cantor: Sire, in order to expand further we must first gain favor of the King
SCG: darn, I've never really got the hang of that tribute thing, guess it will be a long time until i make prince
*goes off and starts gifting gold and techs*