Is it correct to say "We will provide you with the best consultation and assist you filling out..."

the preposition you need is "in"

assist you in filling out..

I don't really like the beginning of the phrase, spelling mistake aside. The adjective "best" does not sound good here.

I might change the sentence to read:

We provide an excellent consultation service and will assist you in filling out....

A change in word usage might help. Because you are already "providing" a consultation...

We will provide you with the best consultation and [additional] assistance in filling out...

The use of an adjective like "additional" is at your discretion depending on the context. However, the use of "in" seems fairly necessary for clarity.