Retail, but as has been mentioned before, I'll take SNES and N64 verticals over horizontals any day.
I do enjoy seeing people's custom covers, but they're not personally my bag, baby. As far as stuff that I'd put on my own games, I mainly prefer the type of customs that present themselves as "alternative art" versions of the original covers/boxes. Like, has proper description, UPC, screenshots, et cetera, but has different artwork and/or layout. I like function AND form.