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.zentralblatt-math.org/zmath/
Also there is Russian http://www.mathnet.ru/ But it is not so developed. But it is free.
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: