Best? I don't know. I would say this, though. I think it's a lot of things, all good. I think it's accessible to many/most shooters. I think it has earned a place as one of the quintessential mid-size revolvers. I think it has earned a reputation for being very, very durable. I think it has earned a reputation as being as accurate as much higher priced revolvers.
So, I wouldn't call it the "best." I'm sure if I looked around I could find one that is better at something than the GP100. But, I think the GP100 is what I said earlier, accessible, durable, accurate, and quintessential. I would call it the benchmark for an "everyman's" revolver. Sure, companies that put a lot more into their revolvers like Colt or the S&W Custom Shop can get more out of it. But, for a stock gun out of the box, I think you're hard pressed to find many deals that give you the same price to performance value that the GP100 does.
I own three GP100s now and an SP101, so I'm certainly a fan. I've also owned a 686 and grew up shooting an old 686 and 586. Looking at the prices of those Smiths now, though, I feel like at my salary level that the Ruger serves me better. I don't think it's better, necessarily. I think the stock sites on the 586/686 were better (not by much) and that it had a better trigger (but not once I got the GP100s broken in). But, these days, those Smiths are often hundreds of dollars more than the GP100, so we're back to price to performance ratio.
The GP100 is objectively an extremely accomplished product at its price point and deserves heaps of praise for that. But, "best" is subjective and is defined in subjective terms. In objective terms, though, it definitely ranks as one of the most successful models of revolvers of all time.