Have finished 3 G&K Games on Marathon level.
#1 Byzantium - Won a cultural victory (Emperor). Their UA can be awesome, and getting it can quickly springboard you light years ahead of your competition. I chose cultural and defensive traits, since my random map type threw me on a panagea. Fought back numerous attacks, allied with a few strong Civs, and coasted to victory.
#2 Carthage - Won a science victory (Immortal). Was thrown in the middle of a huge mountain range on large islands. Which had numerous civs on it. An early war with Genghis but I had a stack of Quins' ready and ended up taking Beshbalik and Karakorum, largely using them. I built my empire rapidly, and didn't have to defend my base cities much as they couldn't get to me without being picked off by a single archer. I didn't found a religion, but my Pantheon belief was 2+ science for each trade route, which I had very early, with harbors etc. Carthage is a beast on an islands map.
#3 Celtics - My first game was with the Celtics (Emperor), I ended up on a continents map. I won a diplomatic victory. I was largely just playing around and enjoying the religion aspect and learning new features of the game. I would have been able to win a cultural or science if I had wanted, but wanted to try diplomatic to fully explore the new city states features.
All were enjoyable. I would probably enjoy the Celts more if I had a semi-set plan right out of the gate. Next up is Ethiopia on Deity. Looks like I am on a Large Islands Map, or possibly a continents map. I plan on just rolling with 1 city the entire game and trying that, though I see the Dutch (who are the only other civ on my island) heading my way with an army, Guess I better buy an archer now. can't wait to get home.