This really isn't a single monster choice, but I'd really like to see devils that actually worked better (and were easy to run) as units that balanced their strengths and weaknesses, as opposed to having lots of random abilities that don't really assist in combat and make them hard to run.
If I was to name a single monster, I'd say the bearded devil (barbazu) -- out of all the devils, this one needs to be simple to run lots of, and still a threat to higher-level characters, since so many of the devils can summon these as backup.
An honorable mention would be the darkmantle, which is supposed to drop on people's heads and suffocate them -- but seems to lack abilities that would let it do so.