By the way, I think it is Shirov.
Who is strongest?
Topalov -ranked 1 at 2796
Anand -ranked 2nd at 2791
shirov -ranked 12th at 2750
Kamsky-ranked 17th at 2725
So I would say Topalov

Topalov -ranked 1 at 2796
Anand -ranked 2nd at 2791
shirov -ranked 12th at 2750
Kamsky-ranked 17th at 2725
So I would say Topalov
I would say Anand for 2 reasons : 1 he IS the current world champion and
2 He has a plus record against Topalov

Of the 4 you mentioned, I think Shirov and Kamsky are distand 3rd and 4th.
We won't exactly know who's stronger until Anand plays Topalov...but at this point in time, I think Anand's history speaks for itself. He may one day be considered one of the all time greats. So he would be my choice.
Topalov must be the strongest
Topalov would be my pick too,but Anand is more consistent.

What about Kramnik ?
The reason I did not include him was because he is passed his prime and is not a serious threat to the world championship anymore. Frankly, I don't think Anand will do much more either.
Which chess player do you think is strongest?
A. Anand
B. Topalov
C. Kamsky
D. Shirov