The 90s WPC games are rewarding because they are pretty tired and dirty at this point if you can find one that hasn't been restored. Parts are available like never before including electronics and resale of restored games is high right now. I also love classic Bally/Stern and EMs. They are a lot of fun to play and parts are pretty easy to come by from Pinball Resource and others.
On the other end of the spectrum for me is anything Data East or pre-LOTR Stern/Sega. Junk design, especially DE, is not fun to deal with. Also, stay away from anything with cabinet made out of particle board unless the cab is in pristine condition and a game that you absolutely love.