Kinda meaningless thread, because 5 replies are going to give you at least 6 opinions, but it could be fun so here's my .02c...
Jointed Rail's payware ATSF SD40-2. SD40s are my favourite US loco, there's just something about the brute functional design with no concession to aesthetics, and imho the skin on this one is about as good as it gets. To my eye both the blue and yellow are absolutely spot-on and there's just enough 'weathering' to look lived in but still loved! It's a big thumbnail but click on it to see the original which is too big to post outside of the screenshots forum...
Andy