I've seen all MLS clubs in person, well, current clubs. I don't recall covering Tampa Bay or Miami but did cover San Jose quite a few times.
But as far as foreign clubs go, I can't say that I have covered too many of them. I actually covered AC Milan when they played Mexico in a friendly at the LA Coliseum right before the 2002 World Cup. I was at the Galaxy-Real Madrid game in 2005 but was NOT at the Chivas-Barcelona match last year. I can't say I was disappointed because I was on vacation at the time, chilling in Michoacan, Mexico.
I've covered the big four Mexican clubs - Chivas, America, Cruz Azul and Pumas - as well as one of the big two Argentine powers - River Plate. I think that's pretty much it. I wasn't at the America-Manchester United match in 2003 in L.A.
After tonight, I'll add Rangers FC to that list and later this summer Chelsea will be there as well.