I am presuming you really mean "Cowboy Del"("Landrys Cowboy Del" is his complete name), and not his sire, "Dels Cowboy"?
I voted, unequivocably, for "Cowboy Del", but I'm guessing that a whole lot of the people now on here don't even know who he is! Sadly, he has NEVER been heavily promoted, in any ongoing way-I knew him in his heyday, and have NEVER, before or since, seen any miniature stallion I liked as well. He has had very limited opportunities at stud, compared to almost ANY other top stallion(and CERTAINLY to the other horse offered in this poll!)-but I firmly believe that on a percentage of total offspring basis, you would find that his shown offspring have done as well, both in halter AND driving(and in other areas!)as about ANY miniature stallion, EVER!
Example: a gelding son of his(and one out of a fairly average mare)is listed on the Jumper Honor Roll(or on top in Western Regional results-forget which place I recently viewed his name---). One of my Cowboy Del sons qualifies as an AMHA Champion, the other has a number of driving points, in limited showing. My guess is that I have seen as many of his limited offspring as anyone besides his (previous)owners, and I have NEVER seen one I wouldn't be happy to own--and I am VERY picky....
BTW,One of the best 'nicks' to Cowboy Del was a Shadow Oaks Paul Bunyon-bred mare-granddaughter, I think. My own Tattoo daughter, who moves very much like Cowboy Del, and also produced "Jess Rocky Rabbit"(back in the show ring and winning AGAIN, after several years of 'retirement"-see Western Regional results, AMHA, 2005), was also a good 'nick' with Cowboy.
Sorry, but definitely neither. If I had to choose it would be Cowboy Del but they are neither one of em my favorite. If I HAD to choose it'd be Iowas Little Kernel or Gold Melody Boy. I also really liked Contender-he was an AMHA National Grand Champion and sired mULTIPLE National Grand Champions but many people have never heard of him