This whole "x is more active" discussion is boring and useless. Focusing on the personality of the community counts more if you are looking for online play.
Here is everything you need: http://doomutils.ucoz.com/load/1
Download all three source ports and then configure to work with this. There, nearly one thousand of servers waiting for you to enjoy, once it is properly configured it will pick the right source port for you to play in a specific server. You're welcome.
But if you are south american and wants to play on low latency, go for ZDaemon, that's the only port with persistent south american servers so far... Tipically, each one is picked up as follows:
ZDaemon if you are south american and/or paranoid about hacks, and prefer a closed source port, i haven't heard of hackers in this port yet (maybe there is, who knows). The mods on the most active servers are map packs, CTF or some cooperative mods that i never really cared enough to try and see what they are. The community used to be really assholish years ago it seems, nowadays it is better, IMO. You will hear a lot of past gossip about the admins being nazi. I say: they aren't, at least not anymore. Ocasionally you will find some laughable smack talk in the main irc channel (note, doomseeker doesn't support this source port, sadly). I have been here for a long time so far, mostly because i have friends nearby who play here, so i'm comfortable with this port.
Zandronum is picked for mods mostly, the "true doom fans" generally plays in passworded servers (priv matches) or in events like FNF, i recommend you to look for people in a clan, these plays, generally, a more "pure" doom, if that's what you are looking for. Other than that, the active servers there are heavily modded (physics, everything!), hence, those "aren't really doom" (GVH, sometimes zdoom wars, zombie horde, all out war 2...). Most of the player base seems to be euros. Expect some retarded assholes here and there, but nothing that can't be taken care of with a good ignore, don't get involved in dramas there and you will be good (don't take this as if they are 100% assholes, there are plenty of good and sane people too). I don't know much about the competitive scene in this one, as they are really spreaded in the channels, so you will have to go deeper in the community to find the "pure doom" players.
Odamex, well... You can't really find anyone to play unless you pop up in the channel for a match, the servers are pretty empty, but most of the player base seems to enjoy the "pure doom", just slightly modded without taking the essence of the game out. I dare to say that it is more like ZDaemon, except that its source code is open to anyone and doesn't have the exp system. Its netcode is very good, possibly one of the best of the three, I did have reasonable matches on high latency here (read 200-300ms). It was the source port of choice for a recent tournament on quakecon and also often picked by community-driven tournaments (IDL I think).
But yea, again, I recommend you to download all of them and configure the source-ports to work with Internet Doom Explorer, that way you can view more servers and players at once.