Best online game though hands down was/is WWII Online but the developers have their heads stuck so far up their backsides they're going ahead with plans that will ruin the enjoyment of the game. Oh well, I was banned for life for being too vocal about it on the forums so if it tanks it won't hurt me.
If you like Medal of Honor, then make sure to check out Call of Duty (if you haven't yet).
What exactly is your problem with WW2O (or rather the future of it)? It's easy to say "developers have their heads up their sixes" ...
N.B. I am not currently a WW2O subscriber since I'm more a flightsimmer and therefore active in WarBirds, but since the WW2O team used to be the WB team I hate to see them disparaged without cause, or at least without explanation.
The best online game I have ever played was Red Faction. As a single-player fps it was pretty good, not really as good as Quake 2, but more advanced in some ways because it was newer.
But in online play it absolutely ROCKED. I had to stop playing it because hours would go by and I wouldn't notice.
More than once, while I was playing, I glanced up at my clock and thought it was broken because it said that 3+ hours had gone by since I started, but it felt like 30 minutes.
It never got old. What an excellent online game. By far the best and most addictive I have played