It depends how you look at it. I often prefer to compare players by surface and the one who has the edge on more surfaces has the overall edge. I do understand why some feel Henin was the best player of 2007 but I still give it to Serena since:
Clay- Henin was better clearly (like nearly every year)
Hard courts- As I said Miami + Australian Open > U.S Open
Grass- I give Serena the edge here since she was badly injured at Wimbledon, hacking a one handed backhand, almost immobile, and still barely lost to Henin in 3 sets.
So since Serena was better on 2 of the 3 majors surfaces I give her the slight edge over Henin as best player of 2007.
USO + Rogers Cup + Stuttgart + Qatar + Zurich + YEC >>> AO + Miami...Henin on Hard Courts.
even if you want to dispute the indoor stuff, Henin still has more titles on Hardcourts than Serena does. Henin played way better at the US Open then Serena did at the Australian, she had a much harder draw to her US Open, straight setted both Williams en route, and won the title without dropping a set (and served each of her first 4 opponents a bagel), whereas Serena at the Aussie almost lost to both Petrova and Peer (she was extremely impressive in the final, but Sharapova also played absolutely terribly). Serena beat Henin in Miami, which was a great win for her, but Henin clobbered Serena in the 2nd set of their US Open match 6-1 and won in straight sets, which since it eventually resulted in a slam win, I'd take that.
As for Grass, If you want to Argue Wimbledon fine, but Henin also won a title on Grass in Eastbourne beating the Reigning Wimbledon Champion in the final in a thriller, so I would still give her the edge on Grass. If you want to say based on Ability Serena is better, ok, but based on Actual results, its Henin, its not Henin's fault Serena only plays Wimbledon in the Grass Season.
Yes Serena spent a lot of time injured in 2007, but Henin was winning virtually everything she entered (except Miami and Wimby) and beat Serena in all 3 of their slam encounters. Now if Serena had won even one of those matches I could see the argument, but she lost all three of them and Henin went on to win 2 of those 3 slams (Wimbledon was just flat out embarassing).
As for the whole year
10 titles > 2 Titles
2 Slams > 1 slam
and undefeated post Wimby
3-0 in slams
3-1 head to head overall
How can you win 10 titles including 3 of the 5 arguable biggest events in the sport and not be the best in the world. You want to go based on on slams sometimes, but now that that argument doesn't benefit Serena....sorry but I just don't get it.