2-Stroke or 4-stroke? In frame or over the rear? (Hah! Fairracing beat me to the punch!)
There are many types of kits out there so you may wish to be more specific. Personally I like the "china girl" two stroke in-frame kits as they have the truest "motorcycle" look (my opinion). Of those kits there's little difference in quality no matter who you get it from but you might want to read some vendor reviews on this site and buy from someone you feel safe buying from. Most of the site sponsors are fairly reputable as sources.
I can't speak for the other engine or mount types as I don't use them.