I used Varathane on my old walnut dining room table about 12 years ago. It
has worn beautifully, still looks gorgeous after a little spritz of polish
and holds up to moisture and spills of all sorts of things.
FWIW, I agree with Leon, only in a little stronger terms than he
expressed. If you are asking this question that means you are unsure
of your method and materials. Don't screw yourself right off the bat
with a quick dry finish, only to thin it again with a solvent that
will make it dry even faster. You will have a mess in short order on
a large surface that you are trying to make smooth.
Since the goal of this is to make the surface as smooth as possible
and not have brush marks, it is important to select the correct
product to seal the top with. Varnishes can tend to be soft, although
there are some hard ones out there. Polyurethanes tend to be harder
and give better abrasion/wear/water resistance overall.
I would find myself a good, normal drying poly and brush it out as
Leon describes and let it dry out for several hours (8 +) then
recoat. The longer work time normal poly provides makes the finish
very forgiving of errant brush strokes and overlaps. The longer lay
out makes the final product smoother and lets it seek its own level a
little better before tightening up.
As always, YMMV.