I think this is true up to a certain point, then stops mattering. Once someone is at the 'pleasant enough to be around' level, I'd much prefer he/she be more intelligent or skilled than more fun if we're working together. This is partly because lower key, quieter, more intelligent people can often turn out to be more interesting and fun in the long run than people who have great social skills and confidence but less depth in their thinking and personalities.
If we extend "fun to work with" to include "quieter, more intelligent" people, I think palish's definition may work rather well. That group definitely falls into "people I want to work with."
Weirdly enough, I just finished reading about how NBA players overwhelmingly wanted to play with Bill Russell over Wilt Chamberlain, as part of an argument that Russell was the better player.
