I'll go with Gary Kasparov. But if Paul Morphy was motivated enough he could have whupped him.
Who is the greatest chess player of all-time?

Currently Kasparov is GOAT. Carlsen can surpass him if he hold the title and highest rated player for the next 10 years.

"The only logical explanation to Paul Morphy being as good as he was in a time where everyone was terrible, especially Steinitz, was that he took a time machine" ~GM Ben Finegold
I tend to agree. Though it's also a question of whether or not the playstyle that he implied would work at all now that players are much better at defending and not accepting gambits.
Another forum to be discussed. Who's your idea? Please share here. I'm just confused and I need answers.