OK, time to cast my vote after reading, riding a few times, and rekindling my love of Nemesis after a brief flirtation with her raunchy dirty sister's dark tunnel in London.
So my answer is:
NEITHER.
No, because this is a stupid question (not a stupid topic, or idea, it's natural so not having a pop at that don't get me wrong). You just can't compare the two. They ride totally differently, the theme is incomparable, they just do different things. Now, if you are to ask which is the most important or quality for their respective times? Nemesis. That's not even debatable, it launched Towers to where it is now.
It's a bit like comparing some shiny new piece of theme, to the Towers themselves. Nemesis, is just part of Alton Tower's, I just marvel at the achievements of near 20 years ago, and if that was shiny and new and built tomorrow in a huge pit totally transforming/creating a new area like Nemesis did, we'd still be excited to ride it now!
The Smiler really had me scared to ride, each time I still get nervous, the loops twists turns and pacing almost becomes more fun each time you ride it - not many rides do that, I actually like it MORE each time, but it just doesn't hold the out and out intensity that Nemesis does, and not one of The Smiler's inversions will ever top Nemesis stall turn and dive into vertical loop then under the tunnel, or roll over the station, or first helix - I could go on...
These rides are just totally different animals.
However, one thing must be said, The Smiler is NOT to the standard it should be and that is a shame, it is far, far too rough for a new coaster and far more so than a ride nearly 20 years it's senior which in coaster terms, I'm sorry, it's unforgivable. Saw has that occasional jolt at the bottom of the lift/drop, but to me, this is FAR worse and comes second only to colossus in terms of roughness to ride. Some of the jolts for a new coaster are just plain awful.
Building team, or coaster design? I don't know enough to tell, but it's one of them, and given Gerstache is not prevalent apparently on many new coasters, and Saw itself I believe is due to some ground subsidence(?), then that means one thing.
If they can fix that, I would be more inclined to consider things further but for workmanship, quality, longevity, creativity and technology respective to it's time, Nemesis - but I cannot argue that, for all it's faults, The Smiler succeeds bizarrely in one thing... it makes me Smile.Always