This has been a good thread. Here are my assorted thoughts that I recall having as I read through it all:
1. The 1500/mile are essentially the same event and have been always considered to be bundled together as one middle distance specialty, so even though technically the mile favors strengths slightly more and the 1500, speed, the two should not be considered separately in any way when discussing the legacy of 1500/mile performances. Thus, I totally disagree with any arguments like, "Well, he hasn't won any MILE races, so he can't be the greatest miler."
2. Centrowitz is certainly the greatest and most accomplished championship 1500/mile runner in American history, which goes a long way towards cementing him as the greatest 1500/mile guy in US history, but being the greatest championship runner isn't everything. It does remain an amazing accomplishment and something for which he should be praised highly and for a long, long time. Also, the fact that the final was slow does not in ANY WAY diminish his gold. Everyone knows championships are weird/slow, and he beat the best in the world, period, which is all championship finals are about.
3. In my mind, the two people he's really competing against are Ryun and Webb. Ryun was absolutely the greatest miler (I'm just going to say "miler" because it's easier to type than "1500m runner") of his time (more on that later), and Webb belongs in the conversation because he is the only mid-distance runner (or long distance runner, for that matter) in the last 30-40 years who can lay legitimate claim to being the best middle distance runner in the world for a period of time, even though that period of time was extremely brief (one season). The Olympic boycott really hurts Steve Scott in this conversation, as he was clearly among the world's best for a long time and if he had medaled or won gold then the conversation around him would be differently. Tragically, it just didn't happen, and we have to go from the data we have. It isn't fair, but it also isn't fair that Ryun and Webb got hurt and, no offense to their coaches/parents, undoubtedly weren't guided with the same incredible skill as Centrowitz who has gotten his dad's coaching as well as Salazar's for his whole life/career. To me, it's very hard to put Lagat in the conversation, and it has nothing to do with xenophobia or not considering him a "real American." I consider him a real American, but do we count the 3:27? Do we count the accomplishments during his time competing for Kenya? It's very bizarre to only count his accomplishments after he became a US citizen. In my opinion, we are comparing the accomplishments of men, and Lagat has been the same man his whole life, so to only count what he did while officially representing America doesn't feel right to me. If we count his whole career, then he is certainly in the conversation for #1 with his 3:26 and multiple Olympic medals barely losing to the fastest runners in history, but I choose not to bring him into the discussion. Cunningham was amazing but it's hard to compare running like 4:05 to today's era.
4. Argument for Ryun being #1: he was the El G of his era. I think some people fail to appreciate what a freak Jim Ryun was back then. He was closing 1500s in sub-50 on dirt tracks when the world was still just beginning to comprehend that sub-4 was possible. He set world records at age 19-21 that lasted for many years. He was completely dominant over his competition, and as was stated before, certainly would have won world championships and probably Olympic gold had it not been at altitude. It's true that 3:33 in poor conditions isn't the equivalent of like a 3:24, but it's definitely a lot better than 3:33 is now. What is it worth? Sub-3:30 for sure. I mean, I watched one of his WR races and the pacing was terrible and he never saw another runner the entire race. Ryun was in uncharted territory. He was exploring new frontiers of what was possible. With today's competition and technology, it's no telling what kind of times he would have put up. Unfortunately, there's no way to know, but the fact he ran times that are still pretty competitive today in the mid-60's is by itself insane. Oh, and he was DEFINITELY CLEAN. Dear Lord. You want to tell me that the guy who ran 3:33 on a dirt track with no pacing on what shoes probably only marginally better than Chuck Taylor's couldn't run with El G if he had been coached by Salazar and had been doped out of his mind? Of course, I don't think Ryun would ever dope, but just imagine if he did, man.
5. Argument for Webb being #1: as has been said by many, he has an American record and he was the best in the world for a brief period of time. It's a real shame he never came home with championship hardware as he certainly had the talent and drive to do so, but for reasons that have been discussed until the cows came home, left again, and came back again, it didn't happen. Webb was also the guy who set these kinds of expectations for everyone. He was shattering that glass ceiling for people like Centro and Rupp. All of that said, even though I am a huge Webb guy, I give Centro a slight edge over Webb due to his consistency over several years, his essentially equal 1500m time, and of course his hardware and Olympic gold beating a really really loaded field.
It's a close call right now. If he were to set a 1500 or mile WR, he would DEFINITELY be #1, but that seems extremely unlikely. As others have stated, the two holes in his resume are that he hasn't set any time records and that he hasn't exhibited dominance against international competition in non-championship races. If he wins a couple of fast Diamond League meets AND sets the AR in the 1500 and/or mile, I would give him a slight edge over Ryun. If he performs very well in fast races over the next year or two, wins another championship gold, and runs 3:27.xx, I would put him definitely #1.