I voted for standard. The game seems to be specifically designed and balanced for the standard speed and map size. The other speeds just have annoying pacing and balance issues for me. The most significant is that damage from combat doesn't scale at all with game speed.
QUICK = Killing units is very hard on quick speed, since they can be built so fast. A replacement unit can be built and damaged units on the front line can be constantly replaced with fresh ones. Early in the game, barbarians can be really annoying if you're a non-militaristic civ. They spawn more quickly but take just as long to kill. On Standard speed, I can usually dedicate one Spearman or one Warrior and one Archer to hunt barb camps. On quick, I need an army of 3 or 4 units dedicated just for hunting barbs because by the time it takes me to kill one unit, 2 more have spawned!
Once you get past the early game, tech progress is so fast that there just isn't time to build and use your units. This gives a greater advantage to defenders, since the time it takes to march an army across the map gives the opponent enough time to research techs and upgrade units. This can restrict the player into having to focus its conquests on nearby civs. The problem with that is that those civs are often my trading partners and friends / allies, so suddenly turning aggro on them hurts me diplomatically with everyone else.
EPIC and MARATHON = Have basically the opposite problem. Units take so long to build that you just can't afford to lose any - especially early in the game. If you lose your first warrior or scout to barbarians, the time it takes to build another one means that all the goody huts are gone and all the CSs have been met.
I'm not saying that the game is broken because of these factors. Rather, they just really screw with the pace of the game for me. It's not so much of a big deal if I know that I'm going for a turtle strategy.
What's really frustrating though, is that I feel that the first half of the game goes too fast and the second half too slow (on any speed). What I would really like is for the first half of the game to play like Epic and the second half to play like standard. That way I have more time to use early game units.