Nobody's quite matches up to Graf's.
The best in women's tennis at the moment are those of Ivanovic, Dementieva, and then further down the list Davenport and Serena because all of her shots carry weight. Last year, Henin's forehand was the best in tennis.
So, Ivanovic vs Graf:
The one area where Ivanovic outflanks Graf is that her forehand down-the-line is a much more natural shot. Graf only hit that shot when she had to, and being the great champion she was, in the moments when she needed to pull them out them bag, she could. But she sometimes lost matches to Seles or Sanchez-Vicario she might have won had she hit that shot at will. Ivanovic does hit that shot as much at will as any player ever has.
That said, nobody comes close to Graf. She hit far more rhythmically than any other player ever has done on the forehand - Ivanovic, Dementieva, Davenport, etc, only accelerate at select moments, when they pick their spot and nail a winner.
Graf nailed ALL of her forehands. There were no lofty, topspun defensive shots. As soon as Graf was able to hit her forehand, her opponents cowered in fear, almost as if waiting for a crack of thunder.
Another thing to remember about Graf's forehand was the angles she produced, the depth she always hit to, and the LOW centre of gravity at which she hit. Her forehands - and backhands, for that matter -- were hit SO low over the net, which gave her opponents less leverage to produce powerful replies.
Yet Steffi could also handle lofty shots to her forehand, as she showed against Martinez and Sabatini. She was able to jump into each shot, both feet in the air, such was her athleticism. She was an unrealised ballet dancer. Her footwork, along with her arm speed, produced the shot.
Graf could always reply with greater depth and power, from depth and power - as Venus Williams found in these clips.
In short, she hit the best forehand ever. Nothing matches up to it.