It's really hard to say.
For starters though, I should point out my experience isn't as extensive as others' might be. I've only been playing MMOs for about 18-20 months. I really only started (per suggestions for other TBBSers) to learn the lay of the land/atmosphere of MMORPGs in anticipation of TOR. I started with three around the same time: RoM, PWI and DDO. Since then I've putted around a good dozen others and at least tried another half dozen on top of that with varying degrees of attention ranging from a half hour to a good weekend.
But one thing I have learned is (and this might seem obvious) all the current F2P games have good things and bad things about them.
All the eastern games follow the pay-to-win model. That is, play the game completely for free but the further into the game you get the harder it becomes to keep up without spending any money.
As for which one is best, well I guess that depends on what you want.
If you want one with a decent pop, you're best bet is sticking to RoM, PWI or FW. The other PWE games and those of other companies are pretty much deserted.
Of those three, PWI is probably the hardest to get into since the community is the most matured. Even when I started, I found it hard to really find others around my level who weren't just PLing alts. And since this was really my first MMO it became discouraging and eventually gave it up. I came back to it later with more experience but decided it just wasn't for me.
The game really focuses on the vanity aspect (i.e. character customization). The actual questing/dungeon crawling aspect it isn't very deep and gets really repetitive. I think a lot of this is because the game caters to girls. Its the one game out there where the female pop rivals the male. In fact, at one time, it may have even been higher.
Also, the game is pretty simplistic. It's clearly meant to be noob friendly and just doesn't offer a lot in terms of game play. It's also kind of set-up to be a rush to end game. It's clearly based on the philosophy that "endgame is all that matters." The early level content is laughable.
On the other hand, the game (at least on an international level) has the highest remaining pop by far. It's also the least guilty of the pay-to-win since most of the cash is spent on the vanity stuff.
FW is basically just an update to it--graphically and gameplay wise. It also flip-flops the emphasis of gameplay vs. vanity. It still has a few aspects of character customization, but nothing like PWI, and the questing offers a lot more substance. However, the emphasis on endgame still remains but in a different way.
Early game doesn't offer much but mid-late game is quite good in terms of content; however, it's all really easy. Then you get to endgame and the difficultly increases dramatically. It's basically "well you've invested all this time and effort into the character, please provide a credit card to continue..."
Another big issue is the game is really prone to server crashes. It's gotten a lot better in recent months I'm told, but still has problems.
That said, the game is the youngest and thus the econ is still mostly stable, so if you get in now you should be okay. And there is enough "fun" to mess around with without having to spend any money.
Lastly is RoM--of the three it is, by far, the one I have the most experience with. It was my primary focus for nearly a year.
In terms of content, RoM wins hands down. The story is (almost) interesting, the quests offer enough variety to keep you interested (and there are LOTS of them) and the instances are fairly good all the way through the game.
It also wins in presentation. The maps are all wonderful and the music (on the whole) is really, really good. Also, given its age, the graphics are fairly decent.
But to fully appreciate it you're going to have to spend some money. The game simply cannot be completed by normal means without help.
For example, the first story-line instance comes at ~level 25 (there are a handful of non-story related ones before that). A full experienced party of that level wearing "normal" blue or better gear will wipe on the second to last boss every time. However, take that same group of people, have each spend a little for stated gear and suddenly the thing becomes 10x easier. Furthermore, if someone spends a bit more to twink he can solo said dungeon by level 30. (Because of the scaling.) This trend continues through the game exponentially until you get to the end and stuff becomes insanely difficult to do with "regular" gear.
There was a time, though, that if one was really diligent, he could get by without having to spend too much by using the in-game resources. Those days are long gone. The whole game took a major dive last spring. And the econ just went completely into the crapper. Stuff was selling on AH for 5x times what it was even three months prior. I'm told it's gotten a lot better as of late but still nothing like it used to be.
As for the Western games, as far as I know, they all follow the pay to unlock content model. That is, only part of the game is actually free. The Turbine games are like this as is AoC and I think DCU is too. I hate this. To me, this really isn't "free."
Now they do offer in-game was to unlock stuff, but usually its more work than its worth. I tried doing this in DDO, but eventually decided the grind just wasn't worth it and it put me off to the game completely.
I've also given LotRO a small try. Most of my TOR guild plays that and I just started in order to socialize. Some of the guildies told me they were to unlock everything for under $100 because they bought stuff when it was on sale. Don't know how universal that is...
One thing I have learned though, is F2P is kind of a myth. In he end I actually spent more on RoM than i would have had I just bought WoW, all the expansions, and a year's sub.
