I've seen Phish quite a few times; they are great in that they always try to bring something new to the table and really push themselves, but they can also end up being pretty rough at times. The recent show in Hampton was a huge improvement over what they were pre-hiatus, and it seems they are on their way back to what they were in '97 and before. I think the improv may be shortened due to the fact Trey is off drugs though, which is both good and bad. They really tried to do too many songs (28!) the first night - I was really disappointed that they didn't save yem for later, and I found it strange that most of the flubs during the 3 nights were made during easier passages while they nailed the most difficult stuff (such as Reba) better than they had in years.
Keller Williams puts on an amazing one-man show. I've seen him about 6 times I think, and he's amazed me since the first time I saw him in Rochester, NY 10 years ago. He's really one of a kind.
As far as jam bands go, I've seen A LOT of them, and the tightest live is moe. Umphrey's Mcgee is a close second. I've seen them both many times and they always impress me. I can't get enough! See you this moe.down.
I would say the best (as in flawlessness) live performance I've ever seen would be a 3-way tie between James Taylor, Eric Clapton, and Zappa Plays Zappa. I expect to add a fourth this summer after seeing the special 2-night Steely Dan concert in NYC, the second night of which Steely Dan will play whatever album wins the most votes from the fans on the first night. I've been waiting for this for so long. The Dan are one of my long-time favorites.
Now only if I could catch Steve Vai, Eric Johnson, and Satch sometime...