Most appealing (?) choices listed first.

Tribunal. Few of the special positions taken, lack of heroes. Plenty of opportunities to shine.
You do trib well, so this is the obvious choice.
Empire: More or less a full council, but it is rare to see more than 3 heroes at any time. Should be plenty of opportunities for intrique and getting fights.
Battle: No commander, decent amount of members across all ranges. With the addition of Nexus and the veil, you always have something to do.
Scion, few members, but some real powerhouses. Scion being scion, this would always be a viable choice.
Outlander, lots of members. No harbinger at the moment. Might be harder to get a fight/shine, but should be plenty of people to interact with in the cabal.
Fortress, lots of members, most leadership positions taken. If you like eq-gathering and joking with your cabal mates, this is a good choice.

I left Nexus out, because well, it is special.