Sometimes singling out a player as the greatest ever may not be fair, or even possible. That said, if I we're to give it a shot it'll be something like this
Golf: Tiger Woods
Football: Messi
Tennis: Roger Federer (men) Serena williams (women)
We all come from different generations though, so it may be unfaair to level someone as the greatest ever. Maybe greatest of our time will be more appropriate @adsactly
The generation gap is one of the biggest issues when having this argument. From the technology of the equipment to the competition and the differences in sports training there are vast differences which make it hard to say for sure.
Exactly. I try not to compare cause there's just no way to measure accurately