Which online service provides the most complete list of citations to given mathematical paper? I mean the citations both in published papers and in preprints. I guess scholar.google is the best, maybe there is something more powerfull?

In my experience google gives you much more than others, but it is not "clean" There are also Those who have subscription (me not) use MathSciNet http://www.ams.org/mathscinet/ Or european analogue http://www.zentralblattmath.org/zmath/ Also there is Russian http://www.mathnet.ru/ But it is not so developed. But it is free. Free: http://academic.research.microsoft.com/ Free: http://www.citebase.org/ Publishing houses like Springer Elseveir provide citation services, but it is mainly within their own journals... 


Apart from the ones Alexander Chervov has mentioned, there are: 


Thought this might be interesting. I became curious and googled for "mathematics citation database". Could not find anything not mentioned here already, except for the following. At http://mathematics.library.cornell.edu/find/MathematicsDatabases they have JahrbuchProject Electronic Research Archive for Mathematics. A resource for searching mathematics literature from from 1868 to 1943. Going there, it is based (I believe) on the Zentralblatt interface, but seems to be particularly sharpened towards old research. They say in particular
Might be useful I think. 

