Anything where the musicians have talent. Thats usually blues & jazz and excludes a whole bunch of rock bands that I like any way for some reason. I am still a metal head at heart but my all time favorite is Frank Zappa. Most of his stuff from about 1970 on is great. The different CD's of live stuff from the '88 tour is the best. I'm sure you Steve Vai fans know that Frank gave him his start. He was a member of his band from about '79-'81. He still shows for reunion and tribute shows.
Metallica (The S&M concert) (No, it's not what you guys are thinking.) They played with the San Fran Symphony, it was great. A wonderful way to break in a new sound system! It is because of their song, "Nothing Else Matters" that I've been working on a novel for the last three years.
Also like Rush, ELO, Derek and The Dominoes ("Layla"), Europe, STP, Ozzy, GnR, .....
Too many favs to list here.
Generally, though, when clients are around, I tend to play soft rock and some classical. I hate having radio on site, the commercials and DJ prattle gets really annoying.