As far as bands I've played with:
-Goatwhore (specifically Ben and secondarily Sammy) were some of the absolute coolest people I've ever hung out with. It was great to play a show with them in fuckin Butte, MT.
-Pathology were killer dudes. Played with them in a tiny ass place in NJ and nobody showed up except the opening bands...so we just hung out together.
-Warbeast, especially Bruce (of Rigor Mortis fame), were very supportive and respectful. They kicked ass.
-Macabre were really cool...I guess I didn't technically play the show, I was running merch...but I'm in the band that played with em now, so I'm counting it.
-Played a show in NYC with Demilitia, featuring Jen Arroyo (ex-Kittie). She was very friendly and supportive.
-Rob Arnold (Chimaira) always seemed like an ass in videos I've seen, but it was pretty cool when he gave me props after they just got done playing...I had played earlier that day.
-Matt DiFabio (Single Bullet Theory/ex-Pissing Razors) hooked me up in a time of need, so I'll always support anything he does.
-Bonded By Blood were very friendly. Their bassist, who isn't in the band anymore, offered to share his fries with me...totally random.
-Merciless Death were cool enough.
-The Headpins (yes, the classic rock band) were nice folks, let us hang in their motel after the show and bullshit.
-Nazareth were fun to hang with, even though I couldn't understand half a damn thing they were saying. I tech'd for a friend who opened for them. It was Dan McAfferty's 60'th birthday, and he still ripped.
-Brian Fair was nice enough, though I caught him when he was waaay stoned and I don't think he really wanted to talk much. He was cool about it though.
-Joe Buck (Assjack/Hank III) did a show with us as a 1-man psychobilly thing...he was fuckin cool as hell, went to breakfast after the show with the band (I had to go to work, so I missed out on that).
I know the topic was "who are dickheads" but I think it's worth more to talk about who weren't.
-The Sammus Theory...they were cool except for their singer was a bit cocky.
-Korn had some crazy shit that you weren't allowed to talk to them unless they talked to you first.
-Matt Zane (Society 1) is hard to be around at times. Edsel Dope as well. I worked a string of shows on a tour they did together. I got along with the rest of the band members pretty well, they were all down to earth except those two.
Sorry...not much of a shitlist.