Well, actually I did the following analyse just for myselft, but maybe it's interesting for someone else.

The most important thing for small sprites is clarity. Maybe in this order: Shape, Volume, Color identity.
* Arne's pure sprite wins in the shape category, the worst one is surt's. Is mainly because the arms are attached to his body and the legs have nearly no shape-information. Hapymonster's is better, but with more definition at the elbows it would be much clearer (and a rounder neck)
* Surt's sprite as a pretty good volume definition, I think. Maybe it's a preference if you like a full defined volume or just a flat shaded (I think in small sprites it's better to go with flat shaded, but however, preference). Happymonster's have nearly no volume information (imho), but more kind of internal lines (which is a bad thing in this small size imho).
* Color wise surt's and arne's are equal imho (just flatshade vs. fulldef.). I don't know the original sprite (just arne's remake), but I think die hands in happymonster's sprite are wrongly colored (and way to flashy).
That were my thoughts about this sprites. I hope I didn't insult anybody.