I think the top few games are mainly ones with poor copy protection. I've played CS with a 123 cd key online, and I know people who have played cracked BF etc. with cracked versions online. ET is free.
lol it's kinda funny... why would people want to play realistic games, if they think that by playing games they can escape the realism and real world for a while .
I know people who say that if a game is unrealistic, it's fun. Probably because it's something new and exciting and a whole new world. My brother thinks that Quake 2 is the best game ever, because it's SO far from realism that it's hard to think adding any cool unrealistic stuff besides the ones already implemented.
Realism is nice, but it gets verrrrry boring after a while.
I remember when i was still in mid-grade(Dutch system) we played Delta force every friday for two hours long.(On the school network, 32 players) It was fun, but after a while it got to the point where it was just a simple matter of who had the best camping spot(Moving was lethal, you could be spotted across the map if you moved).
Now the newest DF-thing, Joint operations, is a lot better. But still.
If the server isn't fully filled or small(Read: Anything less than 64 people) its a real pain in the ass to find a target.
All these games have one other flaw, since they are only suitable for teamplay("One shot-one kill in a slow game" DM is really boring). Because you are killed in one shot, a person with a machinegun in a bunker can do incredible damage. Therefore you need some supportfire from choppers or other vehicles (No snipers, usually they are picked of before they can reach a decent spot) Now try to get people organized on a Public server, its impossible.
And another thing: Most of them are really easy to get very good in. Just stay stealthy, aim before you fire(Most people forget this), support your teammates, and move after you fired a few shots.