That Spring Thomas video is one of his best scenes. He's most definitely the biggest in the industry. I think Mr. 18, if he could get completely hard, would be number 2. I've never seen any gay porn, so I can't comment on men who only star in gay films but I have seen a clip of JJ Wad in the glory hole scene posted here and I'd say he might be number 2.
The one thing about Mandingo that ruins the fact that he's so big is his cumshot. In any fantasy story involving a horse hung male there's always the ability to produce gallons and gallons of cum. Now, understanding that this is fantasy but with guys like Peter North popping huge shots all over the women they work with Mandingo's shots leave something to be desired.