From my understanding (and a little experience) the best F Units are still the Genesis line from Athearn, they are the most prototypical and have a great drive train. Walthers isn't too far behind, but the Genesis shells are the most accurate in terms of contour. For GP7/9s some will argue Genesis, some will argue Walthers Proto. From my research, I believe Walthers is a better product. They capture the contours and details of GP7/9s better than Athearn. Additionally, if you want to customize a locomotive because what's out there isn't available, or you enjoy detailing, the Mainline GP9s are awesome (I have 7 for various projects, and frankly it's kind of addicting). They are the Proto shell minus all the detail, but all the holes for grabs are drilled out. I'd suggest getting DCC ready ones if you go Mainline, the decoder in the equipped models is Lok Select, not a full Lok Sound decoder and I've not heard great things about them in terms of capabilities. Additionally you can fit a P2K shell onto it if you have an older shell that you want to use. You should be able to pick up a DCC ready Mainline GP9 for around $90, and probably lower with Black Friday coming up. Walthers also has Mainline F7s which again are a great product at a great price.
