The best I saw was No Doubt and the worst I saw was Oasis. Liam Gallagher was such an a$$ to the audience that I pretty much quit being an Oasis fan from that day on. It wasn't like their music changed much anyway. No Doubt, on the other hand was a band I actually didn't like at all, but they impressed me with their performance to the point where I bought an album or two. They gained respect from me.
There are many I wish I could have seen. I've heard recordings of Pearl Jam opening for U2 back in 2006 in Hawaii (not sure if they opened for U2 anywhere else on that tour), and it was quite impressive. I'm not sure of the quality, but I would have liked to have seen Garbage when they opened for U2...