Actually it's the opposite, Monfils is annoying, but awesome to watch.. fun tennis everytime he steps on the court (even if he is playing some no talent journeyman).
Personally, the serve bots are the most unwatchable.. Isner, Raonic, Querrey, Karlovic, Rosol, Andujar, Janowicz etc. not because I don't like them as players.. but because I just don't like to watch them play.. always 6-4 sets or tiebreak sets, win or lose, it's plain boring to just watch aces and ground stroke failure.
I also dislike brute hitters like Berdych and Del Potro for the same reason... preferring more agile players like Tipsarevic, Dolgopolov and Hewitt
pushers like Simon can be fun on the other hand, for the amount of balls they put in the court
Any guy with a signature groundstroke (ie fh or bh) can't be considered unwatchable either ,given they can come up with some epic shots.
Also.. clay court pushers like Ramos can be boring too.
For me it's probably Monfils. Similar game to Simon but with worse technique and gameplan.