Other than ThatsDax's engine kits - I'd hafta say the Grubee for the better stuff it comes with... the tensioner has a bearing, the throttle grip/kill switch is a LOT better (a lil more stout and a much better switch design), the clutch cable is nicer (brass cable stop instead of just a pinch screw), the mounting hardware is better (includes both steel and cast clamp plates so you have a choice), the carb is marginally better, and even the stock 415 chain wasn't as bad as others.
The 'Black Stallion' kit is from 'Kings', I got one of those and I hafta say while it wasn't horrible... I wouldn't get another. There was a significant amount of "packing damage" and the accessories included were the worst of all I've seen - for the same price as Dax's or a Grubee.
Another lil tip - if you really want a black engine AND you're picky about looks, get one unpainted and paint it yourself before you put it on the bike. If you take your time with light coats (don't worry about getting it ALL painted in one shot) and use a good engine paint - it'll come out WAY better than the "factory" paint.
They're in a hurry when they paint these things and it shows. Drips, runs, and missed spots are common and I suspect they do the same as I would in their case - they take the worst lookin' motors with the worst scratches, dents, and bad castings and paint them to try and make them look better. It's FAR easier to paint it yourself than to clean up someone else's crappy job (clean it first to get rid of packing grease).
"What is the best V-Frame mount engine I can get for the best price?" ThatsDax, plain and simple lol