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

Hard to say. I saw OMD open for Depeche Mode, as well as the Bjork-led Sugarcubes open for New Order, The National open for the Arcade Fire, and Nine Inch Nails open for Skinny Puppy. Perhaps the oddest was Crispen Glover do a spoken word & short movie montage open for Front Line Assembly.

Some of the best multi-band stage line-ups I've seen:

Pearl Jam, Sound Garden, The Jesus and Mary Chain, Ministry, IceCube/Body Count, and the Red Hot Chili Peppers at Lollapalooza in 1992

The 2006 Voodoo fest line-up included the RHCP, Duran Duran, Flaming Lips, Broken Social Scene, Social Distortion, and Kings of Leon.

I saw the amazing prog band, Renaissance, open for an even more amazing prog band, Gentle Giant at the Shrine Auditorium here in LA. This was in 1976.

Annie Haslam’s 5 octave voice, with near perfect intonation, power and emotion, was always an amazing experience.

At the Santa Monica Civic Auditorium in 1978, I saw the band U.K. (Bill Bruford, Allan Holdsworth, Eddie Jobson, John Wetton) open for Al De Meola.

As much as I love De Meola, U.K. was the better band.

A couple:

In July 1978 went to a Day on the Green in Oakland, CA.  Playing were Aerosmith, Foreigner, Pat Travers and Van Halen.  Opening was a relatively unknow       "punk-ish" rock band, AC/DC.  They came out and literally/utterly destroyed the place.  Jaws dropped.  Not many had seen anything/anyone like Angus before.  The energy was incredible. It was an amazing day.  

In July 1982 went to see The Dead Kennedys, Minor Threat, MDC and the Detonators at the Barn in Torrance.  Opening was again, a relatively unknown band this time from Indy, the Zero Boys.  These kids came out looking kind of dork-ish, (not the typical punk rock "attire" of the day) and just shredded the place from their first song to their last.  All bands were exceptional that day, but the Zero Boys blew them all off the stage.  They were new, fresh and just a GREAT band.  If interested, check out their Vicious Circle LP.  One of the best of the genre.  

In 2000, at the House of Blues on Sunset Blvd, I saw Porcupine Tree open for the French band, Magma.

As much as I love P-Tree, they couldn't hold a candle to Magma. 

Magma was so intense, the musicianship was at such an extreme level, the way their music builds and releases tension and other emotions, they left it all on stage. 

@pdspiegel's listing of Dwight Yoakam opening for Nick Lowe in 1983 reminded me that I saw the legendary songwriting team of Dann Penn and Spooner Oldham open for Nick and his band in the late-90's at The Pantages Theater on Hollywood Blvd., an incredible Art Deco theater. Nick has great taste in opening acts! Dann played acoustic guitar and sang, Spooner played his trademark Wurlitzer electric piano and also sang. Fantastic!

I also saw Dylan at The Pantages, during his five night run there in the early-2000's. He was great that night, and had a killer band. On each of the five nights he had a different opening act; on my night it was Beck, whom I found underwhelming. I wasn't alone---the audience response to his set was tepid.

I had seen Dylan ten years earlier at The Greek Theater, an outdoor venue in L.A. That night he stunk, as did his 3-pc band.