I find these conversations amusing because it really is the nerd (I mean that as a nerd myself; not perjoratively) version of arguing about whether LeBron is better than Jordan (in their respective primes) or whether the GOAT is Brady, Woods or LeBron.

It's just funny that this same argument structure repeats in radically different fields with (often) very different people.

Personally I've never gotten too invested in any of these arguments because they're ultimately unknowable but, more importantly, they're kind of pointless. You can't separate someone from the time they existed in. I was only ever at best average at Chess but even I could recognize that the grats of 100-200 years ago would get wiped out by the modern greats but obviously we know more now, we have better tools now and so on. And you can never really say how a historic great would do in the modern times with modern ideas, knowledge and tools because they're a product of their time.

For Anand it's really obvious that both Carlsen and Kasparov are better given that they've beaten him in World Championship matches (Kasparov once, Magnus twice), have positive head-to-head scores against him and have better other achievements like peak rating or time spent at number 1. There's no metric on which Anand comes out on top in comparison to Carlsen and Kasparov.

On one view, Morphy et al. picked low-hanging fruit, no longer available to Kasparov. On the other, Morphy and Capablanca etc. laid the foundations on which Carlsen and Kasparov walk.