I'm not a big tailgater, I've been to these:
Houston - Pretty cool pre game experience with a country rock band playing the parking lot. Everyone was cool. Lady in front of was gloating during the game until Blue Dredsloe got us the lead at the end for the win. Then she got persnicketty.
New Orleans - People were very cool and welcoming. Of course the Saints sucked then so they couldn't really say too much.
Seattle - In 2004 it was pretty tame. Now, probably no so much. The bandwagon fans are horrible. Opposing fans have been beat up. Cops even had under cover cops in 49er jerseys to catch punks last year.
Dallas - It was pre season a long time ago and I was not yet a Bills fan but my future wife was. We tailgated with my cousins and brothers so it was pretty cool. My drunk cigar smoking cousin did start trash talking to her after the game though.