This is a good thread, but I'd like to add two elements that IMHO dictate your difficulty level: Winning strategies and Map Set ups.
I've won on all difficulty levels but not with every strategy (yet!) or every map setup. I've found that on standard or smaller pangea maps, a warlike strategy can net you a victory in most instances (in my experience the odds of victory go down the closer you are to the middle of the map....being surrounded by aggressive, stronger, faster AI's is a recipe for doom). I set my tech to zero, get as much cash as I can and bring doom on my nearest neighbor until he'll sue for peace and give up all his technology. Rinse and repeat. Makes for a fun game.
Conversely, the same deity level strategy on a larger map doesnt' seem to work (for me at least) because by the time I get to the last couple of opponents, their infrastructure and tech advances put me in a situation where I can't take/defend effectively.

I don't know if the AI gets a culture flipping bonus on deity (of course I don't read the manuals) but it sure seems more difficult to keep your cities loyal on deity...Anyone know for sure?
That said, my personal playing preference is to build a huge civ on a huge map and try to win the cultural battle. Doesn't always happen, and sometimes I have to punt and build that damn spaceship to secure victory, but its still my goal when I set up the game. For that scenario, I prefer Monarch because it seems like the game is fairly balanced and challenging.
I accidentally started a game on reagent once on a huge map(didn't realize it, misclicked I guess) until I got into the late middle ages and discovered how badly I was whippin' every other civ's ass. When I realized my mistake, I quit because there wasn't much of a challenge (to that particular game). I think the AI bonuses (or lack thereof) are very significant.
I'd like to offer some of my own playing advice based on my experiences with different levels. (I apologize if this is outside the spirit of this thread).
If you're playing on Monarch or lower (and have a small or tiny map).
Build out your civs so that your cities overlap more. This will provide you with a more defensible civilization in the early game when you're likely to run up against other civs. When you start conquest on your nearest neighbor (and you should as soon as you have about 5 or 6 cities running, your tightly packed cities will be much easier to defend from counterattacks...
Keep in mind that your probably not going to need the megametropolis to win, regardless of your strategy so crowding the cities won't really ****** you.
If you're playing on Monarch or lower and you've got a standard or larger map, spread your cities out and really, really focus one city on settler production. If the Civ Gods are kind and you can find a cow or wheat square (or more!), found a city and make nothing buy settlers. IF you've played SMAC, think like HIVE. Build, settle, expand, build, settle expand. DO NOT STOP until you're entire continent is settled. If you run up against another civ, pack your cities together more on the border, again, it will make it easier to defend any counterattacks. Plus, if you're using optimal city spread in your heartlands, you'll have enough of the megametropolis late in the game to build wonders/culture points, units, etc.
On Emperor or Deity levels, I suggest starting on a small map. Keep your cities tightly packed, build temples, barracks and units. Move to Monarchy quickly and don't worry about building wonders (most times the AI beats you anyway). Focus on WAR

!!!!
Build up an army of fifeteen to twenty archers, swordsman, horses (whatever resources you can get will influence this) and attack. If the fates are kind, you'll get a GL that you can use to rush build something. When I go to war, I've got a temple and barracks in at least five cities and I've turned over all my production to offensive units (preferably swordsmen and horsemen or UU's depending on the Civ I'm playing). Even if you have a chance to, I wouldn't kill off your first victim....you want him for tribute, technology and (ultimately trading). Just knock 'em down enough that they won't be a threat for a while (20 turns at least) and rebuild your army for the next onslaught.
I haven't really found a strategy on Emp/Deity that enables me to achieve a space build/culture/diplomatic victory. Any suggestions or links to other areas would be greatly appreciated.
Sorry if I've digressed a bit from the main poll "What level do you play on" but I do think it varies based on your goals and the environment in which you find yourself deployed.
Peace.
Corgiman
