I have to say, Crossroads was actually a far better movie than I was expecting. With the exception of the singing bits, it really wasn't that bad. Granted, I was expecting very little, plus I tried Crossroads immediately after Glitter; now THAT movie is pure garbage (The Rifftrax for it is awesome though). But still, I don't really mind watching Crossroads. I rather watch it than either Twilight movie (especially New Moon).
As far as the "Mike only" Riffs go, Roadhouse and Over the Top are the "must haves". Crossroads and Predator are pretty good too. XXX (I love the running gag about everything exploding, but other than that...) and the Fifth Element are decent. I wouldn't bother with Cocktail. If there are any others (other than the Legend Films releases), I haven't seen/heard them.
I agree though that riffing works far better with more than one person. A lot of the Mike and Kevin riffs are among my favorites (eg Casino Royale, The Matrix...). Sometimes I actually prefer two people to three. When it's all three of them it often feels too much like they're just taking turns saying their lines.
On the subject of Avatar, I actually have a background in computer graphics, so I don't mind watching this movie (with the Rifftrax of course; only seen it once without) just for the technical aspects. I watched Jurassic Park six times in the theater for pretty much the same reason.