I would have to say OWO just because they are spread all over the map in a ring. It's hard to get coordination from the fellow tribemates that can be days away. Don't know anything about the actual players for the tribe though, never encountered any of them. Could be full of skilled players, just a weird setup.
Rx2 is the same way but they do have little clusters on the map.
TLG originally was that way as well but they have a pretty nice size cluster in the NW that seems to be pretty strong.
KHN is weakening recently with their leader getting hit hard and people leaving. You'll see the report on the next update.
The rest of the tribes all have giant clusters and are pretty strong.
edit: I'll do the map every few days and the top 10 once a week or if there's a change in ranking.
Based solely upon my personal experience out of the tribes I've encountered I would say Rx2 and next khan, they may not quite be second worst yet but they are on a downward spiral. 7OW recently ran an operation that included taking out the khan leader. I am happy to announce we dropped him to 1 city and he's quit. Several khan have realized whats happening and have jumped ship already.
A good tribe? No not yet. At the moment I'm in TSE, but I'll be leaving soon due to leadership failures and there's an overall lack of drive to help each other.
I looked into LoF, but MAC and TLI are ganging up on them. With MAC advancing southward, I'm going to attempt to build up my forces as much as possible. I hope to be over their minimum requirement if they reach my position XD.