I have to chime in with DEN taking the cake. It's much more comfortable, its layout makes sense (especially the gate numbering - 50 is the middle of the concourse and it goes up or down from there)...
ATL is nice, but it's rather plain, and not very striking from an architectural standpoint. There's no way to compare ATL's A/B/C/D gates with DEN's concourses, and the Jeppessen Terminal at DEN is simply fantastic.
Plus, the bridge from the Terminal to Concourse A offers a unique perspective -- I believe it's one of the few, if not the only bridge that aircraft are authorized to pass under
The transit system is much better at DEN than it used to be -- now it's reliable, and it's fast. ATL's trains are OK, but in my experience, walking can be faster.
Now, as for PDX, that airport has a lot going for it. The reconstruction of the C concourse, even though it's taking for-frigging-ever, will be great when it's done. The new MAX light-rail line will be great, and the glass canopy on the outdoor roadways was simply brilliant and a must-see when you're there.
So, in terms of the US, it goes like this for me: DEN, PDX, ATL, IAH, blah blah blah...
As for Amsterdam, Schipol was a great airport -- they actually had a supermarket!
Again, a lot of thought went into how passengers flow from the a/c to baggage to customs to the train - it all made sense and was easy to navigate. I spent a night there -- 14 hours in the airport total -- so I was sick of it after a while, and saw much more of the ticketing pavilions than I'd like, but such is life... I'd rank it on par with DEN, a little better as far as the transit connection is concerned.