The forehand is the most important shot in tennis, and I'd choose Federer's forehand as the greatest tennis shot ever.
Some say Karlovic got to nš14 in the world and one Wimbledon QF with basically a great serve.
Alberto Berasategui had a great forehand, a horrendous backhand (he always tried to only hit forehands), a horrible serve (he was only 1.72 m or 5' 8'' ) and good legs. He got to nš 7 in the world, was runner-up at RG and won 14 titles. He was basically a great forehand.