My answer would likely be Talk Talk opening for Elvis Costello (Toronto 1982) - had I not arrived late and missed them.
The Clash opening for The Who Billy Squier opening for Pat Benatar Shriekback opening for Simple Minds Level 42 opening for Tears For Fears
There's several that I've been fortunate to see in concert, but I'll start with December 26, 1968 It was in Denver the day after Christmas in 1968, I was home for college break and was lucky to get a ticket, it actually was a Christmas present from my mom First band up was Led Zeppelin, it was their first American tour and their first concert in the States. Talk about getting my attention--WHOA! Spirit was the second band out, their hit single "I Got A Line On You" just hit the charts at that time. They were on for about 45 minutes or so. I had previously bought their first lp at Durango Music when I heard the local station play "Fresh Garbage." Bought the 8 track later at the local Woolworth's. Headline band was the Vanilla Fudge. About 3/4 of the way through Fudge's set people were spent and starting to file out. I stayed till the end. I was up in the cheap seats but I didn't care, I was just lucky to be there. It was just one hell of an evening!
English Beat opening for the Pretenders. Capital Theater 1980 Duran Duran *then* David Johansen opening for Blondie. Brendan Byrne Area 1982
Shovels & Rope opening for DAWES, who I knew nothing about, but they blew everybody away, including myself.
I recall X but am a bit murky on any others, I seemed to recall Siouxie and The Banshees but Dada I am totally un aware of at this show. The opening slot?
English Beat opening for Pretenders at the Palladium Pylon opening for B-52s Central Park My beloved Pulsallama opening for The Clash in Asbury Park Also saw Bush Tetras open for a number bands during the early 80s.
Historic stuff, I knew Zep started on boxing day 1968 but being there not knowing quite what to expect was amazing.
A band called Blazer Blazer. They opened for Acdc in 1978 and were brilliant. We saw them again but can't remember who they opened for.
Status Quo opened for Wishbone Ash on May 1 and 2nd, 1976 at the Ambassador Theatre in St. Louis and both nights they blew them away...and I love Wishbone Ash. By the way, I was not sitting...I'm sure I have other examples, but this would be at the top.
White Stripes opening for Sleater-Kinney, Newport, KY 2000 That said, at the time I couldn't foresee the Stripes every becoming popular... they were just another garage band. I was thinking maybe a modest level of cult success like Flat Duo Jets.
Endless Boogie opening for Sonic Youth at the 930 club. The boogie were not bette than the youth, but different and amazing. Also Kid Congo Powers and the Pink Monkey Birds opening for Nick Cave at the 930...again, not better, but fantastic in their own way and made be a big fan.....