Here's my side. You judges better pick me, or I'll haaaaaarm yooouuuuu!
Joe Besser Vs. "Curly" Joe Derita. Picking between the two is indeed like choosing between a hot stake or a cold chop as a means of death. 99% of all Stooge fans will agree any great Stooge lineup consists of a Fine and two Howards, and those Joe guys were simply plugging away past the group's prime. Nobody will debate this point, but what is up for debate is who was the better Stooge of the two. The answer is Joe Besser, the far superior comedian who added more to the Stooge vocabulary in his brief stint than Joe Derita did in over ten years.
Before joining The Three Stooges, Joe Besser had already developed a comic persona as a squeamish, child like pansy. Loud noises frightened him (Not so LOOOOOUUUUD), he was sensitive to touch (Not so HAAAAARRRRRRRD), and he had a fey, whining voice that could easily pass as a spoiled child's. In fact, his persona was so child like that he could convincingly wear a Buster Brown sailor suit and play a child called "Stinky" on the Abbott and Costello show. Say what you will about Besser as a Stooge, but as "Stinky," Joe Besser could hold his own, and arguably even steal a scene from Lou Costello. Besser had the high octane delivery that was necessary to both jab with Bud and Lou and to be a third Stooge.
Joe Derita, based on his pre-Stooge filmography, showed very little in terms of a unique comic persona. In his Columbia shorts, he was a soft spoken Lou Costello lookalike who in one short (SLAPPILY MARRIED), performs a Bud and Lou like verbal encounter in the Costello role. He fails to summon a fraction of the energy or the laughs Bud and Lou do in such routines, his soft spoken style not quite as suited to such material, nevermind being the third Stooge.
Joe Derita's strong point as a Stooge was that he could handle all the stock comedian moves and he was professional enough to keep Moe and Larry going throughout the 60's. The man can take a great pratfall for his size and could do a double take, but these are moves all generic slapstick comedians should be able to handle.
Joe Derita lacked any individual quirks that make him memorable as a character. The Three Stooges have always employed strong characters before him, and each Stooge is simple to impersonate. Moe has his sugar bowl haircut, his assorted slaps, pokes, insults, and tough guy voice. Larry had that wonderful frizzy hair, sandpaper voice, and was one of THE great reactionary comics ever. Curly had his bald head, squeaky voice, his nyuk's, his woo woo woo's, and his graceful movements. Shemp had his unwashed hair, his eeb-eeb-eeb's, his shadow boxing routine, and a herky jerky personality. Derita? Like I said, he could handle all the stock comedian moves, which is why he was able to be a comedian for so long, but he was simply the guy who showed up to work and did no more than what was required. He had no individual character whatsoever, and if it weren't for Moe and Larry, he'd be joining the Brown and Carney's or Collins and Kennedy's of the world in total obscurity.
Joe Besser, as much of an acquired taste as he was, had his own unique persona just like his third Stooge predecessors. His previously mentioned "not so hard" and "not so loud" sayings are a part of Stooge lore along with his fey voice and his limp wristed slaps. He may have been opposed to being slapped and gouged (though it should be noted as the shorts progressed, so did Besser's slapstick involvement), but he certainly was memorable enough and had the energy to keep up with the boys. By the time we get to OIL'S WELL THAT ENDS WELL, he even performs a slapstick bunk bed scene with Moe and Larry that can be considered classic.
Ultimately, we love The Three Stooges for their personalities. Sure, we love the slapstick too, but there were hundreds of film comedians performing Sennett like slapstick in the early days of screen comedy. The majority of them lacked any memorable personas, forcing the slapstick to become a tired cliche. The reason why The Three Stooges brand of slapstick works was because of the personalities that went into it. Joe Derita was simply a professional, stock comedian who was lucky to work with Moe and Larry. Besser's persona was either love or hate, but he at least had a screen persona, the very thing that separated The Three Stooges from the lesser comedians. He's not Curly or Shemp, but Joe Besser at least adds something to the Stooge experience, and for that reason, he is the greater Stooge than Derita.