The best opening act you've ever seen & heard?

First warm up band that blew my mind was a little band from TX, before the beards, wearing white cowboy suits and hats.  Never heard of ZZ until that night in Tulsa.  Main act was Humble Pie.  IIRC, it was summer of 72.  

A more recent warm up act that endeared me forever to her music was Shawn Colvin, who preceded Don Henley on his Cass County tour earlier this century.  It was at the Cobb Energy Center in GA, and her acoustic guitar was as good as I've ever heard live.

Most disappointing warm up band (and please don't bash me, they're very talented) was Tedeschi Trucks, who filled in for Steely Dan when illness took them offline as warmup for the Eagles tour with Vince Gill.  Or Vince Gill's tour with his new band.  TT was just not what my ears were looking for, given my original expectation was for SD.  The Eagles were better than ever that night, so my disappointment was short lived.

1972 Irvine Auditorium on Penns campus - Little Feat (with Lowell George) opened for Captain Beefheart.

Somebodys bassist was MIA so either Little Feats bassist played with Beefheart or vice versa.

In the Summer of '68 my best friend and I went to The San Jose Civic Auditorium to see Lee Michaels open for Steppenwolf. The curtains opened, and there was Lee sitting at his Hammond organ and dual Leslie speaker cabinets, with about fifteen Vox Super Beatle amps running from left to right at the back of the stage. Impressive!

Lee and drummer Frosty kicked off the set with one of the songs from his Carnival Of Life album, and they sounded fantastic. A roadie brought out a glass of some liquid for Lee, setting it on his organ. Unfortunately the kid wasn't looking at what he was doing, as the glass fell over, the liquid splashing all over the organ, shorting something out. End of set!

We stayed for Steppenwolf's set, but I can't tell you much about it. The only live show at which I fell asleep.

This isn't technically a case of an opening act, but at the 1968 Northern California Folk-Rock Festival I witnessed The Electric Flag precede The Doors on stage. The concert poster shows Country Joe & The Fish as the act between the two, but I don't remember them (didn't like 'em).

The Electric Flag were absolutely on fire, Michael Bloomfield proving what a great guitarist he was, drummer Buddy Miles making Keith Moon seem like a light-weight. The Doors had to follow them, and in contrast sounded pretty weak. I had seen the Doors about six months earlier (at The Continental Ballroom in Santa Clara, where I played my first live show in 1966), and they were much better that night. Maybe hearing The Flag intimidated the members of The Doors.

Mid 2000s...Joe Satriani opened for Deep Purple..made Deep Purple look a lil weak thereafter.