We've used the Minwax poly (NOT the water based version) with great results. I'm partial to the satin finish (low sheen) but you may prefer a higher gloss, which they have as well.
I'll usually put 3 coats of poly on, sanding VERY LIGHTLY between coats (after it has dried, of course) with either 220 or 320 grit sandpaper.
What you're trying to accomplish with the sanding is to knock any nibs (small bumps) off the finish coat before applying subsequent coats. DO NOT sand through the new topcoat, just enough to knock down the small bumps.
By sanding LIGHTLY between coats, your finish coat will be nice and smooth to the touch, which is important on a handrail.
P.S. Can you guess that I may have sanded a little too aggressively the first time I did this?