The only player that could be #1 today is....
Novak Djokovic himself....oh wait, he is #1
I think Agassi would be like a Murray in today's game except possibly better because of his somewhat strong mental game.
So, id say him and current Djoker would be equal
Then again, it would be a 4-way 1st place tie between
Prime Djoker, Fed, Nadal, and Agassi
They're all great defensive players so really it's a toss up. I think they would rotate titles with Fed winning the most because he's the most versatile then Djokovic and Nadal would be undisputed clay king still