Four different men sat at the desk across from Andy over the 12 years of the show, and each partnership had its own flavor, its pluses and minuses. Here's my chronological look at all four:
But as Andy sobered up and became a good cop again, the two partners worked together less and less, to the point where they were rarely involved with the same cases by the end of the first season. Why? Blame Caruso, who went more than a little nutso at his overnight success and began to chafe at the increasing spotlight being thrown on Franz. It got to the point where if a John/Andy scene wasn't explicitly about John, Caruso just sleepwalked through it. So the writers deliberately began splitting them up as the first season went along.
The funny thing is, Kelly was the best character to ever work alongside Sipowicz, and the worst partner. As written by Milch and played by Caruso, he was more complex and clearly-defined than any of the guys who followed. (Bobby Simone barely even existed on the page; Jimmy Smits had to make up whatever was interesting about him.) But because of Caruso's temporary bout of megalomania, the team never would have lasted, even if David hadn't quit to do movies.
Like Stan and Chet, Newman and Redford, Laurel and Hardy, this was a partnership for the ages. Like I said above, Simone was never much of a character, but Smits and Franz played off each other just about perfectly. It was a study in contrasts: bald, fat, tacky, hot-headed Andy paired with tall, hunky, fashionable, introverted Bobby.
Chemistry isn't something that can be planned, or even properly described sometimes. It's just something that happens. Watch the "Duke of Earl" duet, or the "Closing Time" scene where Bobby asks Andy if he wants help; those were virtuoso duets.
Bobby was also the only partner whom Andy viewed as a complete equal, and vice versa. He and Kelly had too much history together (Kelly as former student and then witness to Andy's drunken fall), while he played mentor (or tried to) with both Danny and John Clark. After an initial rough patch where they disagreed over petty stuff like sodas, these two fell into a groove and stayed there for four years.
Squandered potential, thy name is Danny Sorensen.
When word came out that Bochco and Milch had cast the kid from "Silver Spoons" to replace Jimmy Smits, the reactions ranged from "Are you freaking kidding me?!?!?!" to... well, that was pretty much the only reaction. But Bochco and Milch knew something we didn't: Schroder had become a really strong actor while no one was looking, and he was more than capable of standing next to Dennis Franz.
And at the very start, the writing of Danny played perfectly off our expectations. Sipowicz rolled his eyes at this "14-year-old," Diane resented the hell out of him for daring to replace her (and our) beloved Bobby, and Danny had to go balls-out to earn everyone's respect. His first episode, "Danny Boy," was the best introductory episode for a character other than the series premiere.
Had the show stayed this course, I think we would all look back on the Danny era much more fondly. But it seems like Milch, having established Danny's bona fides in a single episode, got bored with that approach and went in a new direction. That confident, smooth-beyond-his-years Danny who could take control of a crime scene or interview room without breaking a sweat? Gone. In his place came the amazing Paperclip Man, hero with a thousand neuroses!
Hell, even that might have worked, if Milch had bothered to explain the source of Danny's traumas in a timely, coherent fashion. But he dragged out the mystery for two seasons, then left without apparently telling anyone what the master plan was (assuming there was one).
And the idea that Andy viewed Danny as a surrogate for Andy Jr. was another promising angle that went nowhere. Basically, for three years these two worked together without ever really developing any kind of rhythm. I can't fault the actors on this one; the writing just was never there, either for Danny as an individual or as Andy's partner.
In a way, Junior was a Greatest Hits collection of Andy's three previous partners. Like Kelly, he was a second-generation cop named John Jr. with an ADA girlfriend named Laurie. Like Simone, he was handsome and had a thing for rescuing damaged women. Like Sorensen, he was a 28-year-old rookie detective out of Narcotics who had the unenviable task of replacing Andy's dead partner.
But John became his own man, mainly because the writers had a clear idea of who he was and where they were going to take him. Where Milch abandoned the teacher/student dynamic between Andy and Danny in no time flat, Bochco and the later writers held firm to it. The John Clark we met at the start of season eight didn't really know what he was doing, but he was eager to learn from Andy; the John Clark from "Moving Day" is clearly and credibly the squad's lead detective.
I do think that the writers occasionally moved Junior ahead too quickly. His best moments came early on, when he was screwing up through inexperience (losing his shield, bungling the James Kilik situation), but Mark-Paul Gosselaar was such a natural in the part that I suppose it was hard to make John seem like a rookie for too long.
1) Bobby Simone: Not only was the strongest and most likable of the bunch, I think he provided the greatest contrast to Andy, and was (though probably not intentionally) Andy's greatest teacher in terms of how to get along in life. He taught by example. Simone--often referred to as St. Bobby--was an enormously positive influence in Andy's life (on par with Sylvia), and the tension and drama that resulted in Andy's struggle to be a better man was never more clear than it was during the Simone years. Add to that the natural and very real chemistry between Dennis Franz and Jimmy Smits, and you have a perfect pairing.
2) John Clark: Yeah, I put him above Kelly, though I know many others wouldn't. Here's why: By the time the series got to it's final few seasons, Andy's evolution from a racist, whore-screwing, perp-beating, snot-flying drunk was pretty much over. As the Sorenson years will prove, there wasn't much more conflict to be mined in that field. It was past time for something new, and John Clark was it. Like Simone, he didn't have too many demons, but unlike Simone, he wasn't there to teach Andy anything. He gave the New Andy a foundation on which to grow--Andy could protect him without having to fix him. As played by Mark-Paul Gosselaar, Clark was fresh, strong, straightforward, and his addition to the cast breathed new life into the show at a critical time.
3) John Kelly: We hardly knew him, really. I put him third because the character of John Kelly was topnotch, and the first two seasons of the show with him were really outstanding But as good as it all was, Kelly is a character who could have existed without Andy, and in the big scheme of things that doesn't make for a very good partnership. I wonder how long the show would have gone on and been good had David Caruso stayed. I also think that for as great as Caruso was in that role, there's no question in my mind that Smits and Gosselaar have it all over him in acting in general.
4) Danny Sorenson: A mess. No fault of Rick Schroder's, this character was about as clearly defined as pudding. His impact on Andy was nil. They started out trying to do a father-son thing, but that never worked and then it was changed. Danny's backstory was never fully explored, his relationships were beyond screwed up and he ended up being more of an emotional mess than Andy ever was. There just wasn't much about Danny you could find to love.
5) Occasional pairings: I always enjoyed it when Andy teamed up with someone else in the interview room or out on the street. Andy and Medavoy together always resulted in an amusing moment or two, and gave us a chance to see Andy being less than patient with someone (Greg) who didn't deserve a beating but to whom at times he'd have gladly given one. The best of these occasional pairings, though, was Connie. If she'd have ever been Andy's partner in the long term, I'd probably have her name up there above Bobby Simone. It was interesting, engaging, fun to see her match Andy wit for wit, comment for comment, brave step for brave step. It's too bad there wasn't more of it.