Manifold

Manifold purports to be "the best GIS ever". Who am I to argue? :-) The price is certainly good.

I've worked with ESRI ArcView (3.x) and ArcGIS (8.x/9.x). I like ArcMap. I will reserve further comment for a later time.
I tried GRASS and it didn't really set my world on fire.
I looked at TNTmips. I gave QGIS a shot. I've tried Delorme TOPO! and Garmin Mapsource but they don't really qualify as GIS systems.

So I guess for me this means I have to compare it to than ArcMap.