For me, my best gig has to be Thin Lizzy at the Marquee in the early 70's. A rumour went round that Gary Moore (then of Skid Row) was filling in for Eric Bell who was ill. I later heard there was around 900 people there in a club that was only supposed to hold around 300. Fantastic atmosphere.
A close second was seeing a then unknown Clannad performing in the foyer of Wandsworth Town Hall to an audience of no more than around 20.
Worst gig has to be Devo at the then Hammersmith Odeon that a friend persuaded me to take my girlfriend to. Spent most of the concert watching them on a screen before they finally graced us with their presence on stage for a whole 20 minutes! What a pile of crap they were too.