I'm at 94 countries myself. I went to Belarus lat year, it wasn't that difficult. I think I got a visa support letter from a hotel or travel agency, sent that in to the embassy with my passport (using traveldocs.com) and got the visa a week or two later. Americans can also get visa on arrival at the airport, but it costs $400 or so, and you probably need the visa support letter. Minsk is well worth visiting though.

I have heard that Iran is a lot more difficult for Americans to get a visa, as of this year. I have also heard that Angola is not easy.

Aside from visas, I think the hardest to visit (for Americans) would be some of the little Pacific island countries. Infrequent, expensive flights.

You can all feel free to keep me posted if you want someone to join you on a silly country run.