Line 6 or Sennheiser. Both sound better than the comparably priced Shures. Still, neither will sound as good as the wired version of the same capsule.
Having used a Shure PG58 and a Sennheiser Freeport on the same night through the same system, I have to say the Shure has a better sound; it's no SM58, but it's very usable. No experience of the Line6 $300(ish) line.
As for the higher priced stuff:
At my church we use a Sennheiser system with 4 G2s and 4 G3s (gradually upgrading), all with the 500 heads. They were completely blown out of the water by some Shures we hired in for an event.
IME/IMO, unless you're willing to pay top dollar for the 900 heads, you're better off buying something other than Sennheiser wireless gear. We've tried the new digital Line6 stuff ($600ish range), and it rocks. The stock capsules with the 58beta preset sound almost the same as the real thing (direct A/B comparison), although the feedback rejection is *slightly* less impressive.
As for Shure, we've tried them for a few events/conferences; although I was impressed, there wasn't really time to do any tests or form opinon other than 'these mics are good!' I can't remember which system we were using, I think it was the one below 'top of the range' with the 58beta capsules.