This is a pretty good question and I think it's been asked before on this forum in a few different threads and my ratings are as follows:
2015: rtcll, stork, sleepy, jag420
scuzzle does some pretty good capping and if you put regular community people like chutia/nav0x/driftwood into play for capping they can do some pretty good camping runs which can make the game harder for the other team
I'm not qualified to comment on who was best in Tribes NA LT because I've never been a main part of the community.
I can say who was best in Aussie LT/base back when it was still going and I would love to answer that question but unfortunately most people in NA wouldn't be that interested
What really kills me is that Tribes is pretty dead now so there are very few NA players left. It also doesn't still have its regular Aussie/Euro counterparts. In general if Tribes were still relatively active, we could have international matches where people from NA would play Aussie teams and Euro teams etc. There was plenty of international talent on display, I couldn't speak for the Europeans but the Aussie side would have some decent cappers/LD.
In the end the question is why is there a lack of players in the game now, I don't believe anything can be done about it. IMHO Tribes has had its heydays and the switch to T:A moved a lot of people over which was all justified.
If there were some kind of alternate reality where Tribes thrived more than it does today, I would envision something like a huge LT following/Aussie GameArena/extra comp server, Euro and Aussie players, a revival of hackington, and like I said more international-type matches.