I voted Improved Grapple. If you are stuck using core rules like I am in a new 3.5 game, it is the better feat. +4 bonus on all grapple attempts is pretty good.
(If your looking for a visual grappling does not HAVE to mean wrestling) Wing Chun, Aki Do, Any "Jitsu, Tai Chi, Chin Na, and many other arts use joint locks, and other methods to impose pain/damage. And most of them are not on the ground.
Stunning Fist is only usable so many times a day and the save is rather easy to make. In my experience it wasn't very usefull. If you go with OA though you will need it for a number of other feats as a prereq. SOME of those feats could be re-written to use Improved grapple to fit the feat better, but others like Pain Touch are more like enhancers to Stunning Fist.
I would only take stunning fist if I needed it for OA feat chains.
It kinda stinks being limited to the PHB for monk feats though.

But hey just make leamonade.
I also prefer high STR monks ONLY FOR GAME MECHANIC reasons though. If the rules supported it I would prefer a high WIS / DEX build. But alas the core rules don't support this very well. At least I rolled good when making my monk!