Seaking
Uncharitable Spy
The syndicate. All their support classes are too good.
Don't think -ts- is a team anymore. Lots of members of their teams are split up amongst other plat teams like Ginyu (who will win this coming season) and Deathforce9 (who will go 2nd or 3rd this season).
Personally, my favorite HL team is bpm for plat. For 6's I'd probably go with Car Shoppers in plat, but it doesn't matter because everyone will be destroyed by Max4heads. Which leads me to dabes team: HRG4lyfe.
If skial forum regulars are interested in following a team, Ruelel, Twilight and I are on a Secondhand Mercenaries in HL Silver. Not sure if there's any other mid-upper level teams in UGC with many forum or server regulars on it, but hey, that's two Skial admins right there. There should be a few skial regulars in steel and iron though.