1. Type O Negative - twice. I saw Jaegermeister 2008, which was okay, and the last show I saw was their third-to-last show ever, also one of the best setlists I've seen. I was never more excited for a show than that one. They were (and still are) my favorite band of all time, and there's nothing like seeing your absolute favorite band in concert.
2. Danzig - three times. I saw the 20th Anniversary Tour, DRS 2011, and the Legacy set in Chicago, best show ever. I never thought I'd have the chance to see him, with all his nonsense about retiring, so I can really appreciate the shows I've seen..
3. Roger Waters doing Dark Side of the Moon in 2007. I know he toured pretty extensively for this, but with him doing The Wall now, DSOTM will never happen again.
4. Gojira. Three times. The first time was opening for Lamb of God in 2007, I hadn't heard them prior to that, and they blew me away. Then I saw the open for In Flames in 2008. Last time I saw them they headlined a bar in 2009 on an off-date of the Death Magnetic tour, it was one of the best shows I've ever seen.
5. Napalm Death - 2008, opening for Devildriver. they haven't toured the US in a while, kind of a life-changing performance.
6. Slayer - four times. 2006, 2007, 2009, 2010. I got to see them before they stopped playing songs from Hell Awaits, but, it was when they stopped doing real headlining shows in the US and started co-headlining every tour. But, they were all great shows, especially Unholy Alliance 2006, which was also my first GA metal show.
7. Megadeth - five times. Gigantour 2006, 2007, Gigantour 2008, 2009, 2010. Best show was the RIP tour, one of my favorite albums of all time.
8. Metallica - twice. 2005, 2009. Both times were great, but the first time was pretty unique. 11/15/05.
9. Immolation - 2010. My favorite death metal band. Even though they've been touring steadily for the past two years, they seem to have pretty poor attendance at their shows, which makes me worry that they'll hang it up after the next album or two.
10. Cynic - 2009. I saw their first US tour post-reunion. I remember it seemed liked like forever since I started liking them before I finally got to see them. Unfortunately, I haven't seen them since, but I'll try to catch them this November.
9/20 - Overkill or Nik Turner (or both!)
9/23 - King Crimson