Falcon BMS 4.33 is a pretty intense F16 sim (other aircraft are also available), takes a while to get into though - much more detailed and realistic than DCS (the avionics and flight model have been updated and tweaked by real F16 drivers to be as real as possible sans Top Secret functionality - and even some of that stuff has got in there). BMS (Benchmark Sims) update to the original Falcon is free, but you need to have the original game which you can get from GOG Games for about £10. THe single player Korea campaign is pretty good, it's one of the best Dynamic Campaign engines that there has ever been.
WWIIOL is one of the oldest MMO's out there, again a steep learning curve but plenty of PvP if you like your WWII stuff, it is subscription based but there is a free 2 play account available with limited equipment (Hurricane mk 1 versus veterans flying FW190s!) of course you can also pilot boats and tanks and run around with a rifle if you feel the need. A single server (no instancing) and half sized terrain of northern europe and specific Attack/defense points can make for some pretty cool battles, personally one of the greatest things about this game is tootling around in the skies behind the front line and spotting a player driven truck pulling a player controlled anti-tank gun who have been driving for half an hour and strafing the snot out of them, or dropping a bomb on a building full of players and going home with a 12 kill mission.
Somewhere between the two drops IL2 - Cliffs of Dover. The original game was pretty buggy, but a group of modders called Team Fusion have taken the base game and made a pretty solid PvP WWII flight sim out of it, again its very realistic and some of the dogfights I've been in have been pretty intense. there are several servers available, one of which I think is a semi-persistant world, where the Jerries are bombing the airfields and radar stations and the plucky Brits are defending them.
Lastly, if you like your aircraft made out of string and paper, there is Rise of Flight. If I recall correctly, it's free but you pay for extra flyable aircraft, again plenty of online servers and again some pretty realistic air to air combat - to the point where you can fly up next to an enemy aircraft, take out your Webley pistol and shoot the pilot (or indeed turn around and shoot your gunner!)
These four flight sims are my go-to games for online pvp - realistic flight models and real world tactics can have you either sweating buckets in a turn fight on the deck, or hanging lazily from your chute wondering what just happened when you have been jumped (beware the Hun in the Sun!) and you weren't checking your six!
https://www.bmsforum.org/forum/forum.php
http://www.battlegroundeurope.net/
http://theairtacticalassaultgroup.com/forum/forumdisplay.php?f=91
http://riseofflight.com/
(other, less realistic flight sims are also available

)