there is just so many options available it's impossible to say which is best.
I've been looking at the GSPORT Ribcage. Honestly..... I think I'm just going to buy a complete bike. Just building a set of rims, I'm spending 300 plus dollars, WITHOUT using the Profile ZCoaster. I'm concerned that the S-Kill or the Darwin will be not as sturdy as I'd like. I can't find a place to order a KHE Switch.... baring a hookup on the KHE, I have been eyeballing the St. Martin for about 2 years of wanting to get back into flatland.
Just to clear up the KHE quality comment,KHE has always had an extensive line up of complete flatland bikes, from beginner to high end. In the last few years it has been cutting down the line and this year they only have 1 mid-range complete. (which I think is pretty good for an intermediate rider).