The best rivalries are NY, New England, Chicago, Columbus, and LA Galaxy (all are original MLS teams and the hate runs deep). Even though they are a new expansion team, there's usually some good hatin' with Philly for obvious reasons. KC is an original team, but no one really cares about them. Seattle usually has a good atmosphere (a surprising number of their fans travel all the way across the country), ditto Toronto. Dallas, Salt Lake, Houston, Chivas, and San Jose games are usually boring.
We now have Vancouver and Portland (since last year), and now Montreal in the league...I'll bet a lot of Montreal fans will make the trip down, as the team had a big following in the lower leagues.
[edit: and if you like to hear teenage girls screaming, David Beckham is with LA Galaxy for another year]
Who do you recommend? Let's say three to five games at RFK.