Worst live show I saw was Hell is For Heroes (or more accurately one of the support bands, We Will Be Pilots, who were so mind-bogglingly pretentious and boring I was trying to slit my throat with a plastic beer mug by the end) and second worst was Dillinger Escape Plan, who sucked muchos horse cock. That evening was only good in any way because Ephel Duath were the first support, and they tore. it. up.
As for dissapointments...SikTh were pretty lackluster live, they seemed very sort of subdued compared to what I'd been led to expect, and didn't play my favourite song (How May I Help You?), and I remember being kinda bummed out by Slayer live, though in fairness to them, I was 15 when I saw them and I didn't own a single Slayer album, and it's always worse if you don't know the songs.