If you could, at least pick one of the main 4 that already have a forum now, and then pick one that isn't the main 4.
If you were going to swap to that clan anyways, that kind of ruins the point of the topic, but you can post anywys if you feel like it.

I have a strong bias for Jade Falcon from playing Mechwarrior 2 when I was a kid.... Also, they seem to like heavy mechs (not assaults), and jumpjets a lot. Both of which I like a lot. Hitting hard and fast is something I can appreciate.
The main flaw for Jade Falcon is that they were basicly portrayed as evil bad guys in the battletech cartoon.

While wolves are cool, Clan Wolf seems boring to me outside of the fact they are called Wolf.
Smoke Jaguar isn't all that interesting to me either. However, they did design the Mad Dog (my favorite mech in battletech).
Ghost Bear use the Mad Dog more than other clans so that gives them a big + for me.
However they seem to be the Steiner of the clans in the fact that they like to have really large mechs. Though some people have told me they like to have lots of super heavy mechs for their main force, while using some support mechs (specificly Mad Dog), and I like playing support while having my team is being a bunch of big heavy mechs.
My knowledge on the clans outside of the main 4 is more limited but :
Nova Cat. They have my 3rd favorite mech in battletech the Shadowcat. They seem to love ER Large Lasers, which is pretty close to PPCs (my favorite weapon after all the missle weapons). Sadly there's a bunch of little things that bother me about Nova Cats. Shadow cat is awesome, and Nova+Nova Cat are fun to play from time to time, but that only goes so far.
Goliath Scorpion. Speed with Precision, Zellbrigen honor, they try to research star league stuff. Unfortunately, the idea of them being addicted to toxins to help them find star league stuff is rather uncool. This is the most likely one outside of the main 4 clans.
Cloud Cobra has a strong emphasis on Aerospace, and I love being in the air. However, since we are piloting only mechs in MWO, they aren't the best, cause I don't really like most of their mechs.
Coyote: While the idea of Zerging with a ton of mechs sounds immensely fun, MWO isn't the game for that. They made Advanced Tactical Missles (ATMs) and I love missles. However outside of that they seem like pretty boring clan.
I'd have to do a lot more research to decide between Goliath Scorpion, Ghost Bear, and Jade Falcon. I'd probably pick Ghost bears first, then Jade Falcons, then Goliath Scorpion.