There's so so many that could be included in this, and I see that they've all been named in other posts before including my choice below. But if i had to say one, just one, player that was a joy to watch and I finally got to see him 'in the flesh' in his last year at Wimbledon was, Fabrice Santoro. Always Santoro. Only Santoro.
He wasn't my favourite player, I grew up loving McEnroe, then Agassi, now it's Tsonga and Berdych. But, you just had to watch him if you could. Even if you ask the professionals themselves, I would say a good 75% of them would say Santoro.
He played in such a way that was unnerving if you were on the opposite side of the net. I never have been, never would have got close to being on the same court as him but it would have been so much fun just trying to win points against him. You know what you're in for but powerless to do absolutely anything about it. You don't get the name 'The Magician' without a trick or two . . . Genius.