I find a lot of breeds are sexy for different reasons, some of it being more of a stronger sexual attraction than other breeds which are more of a romantic/platonic attraction. A non-sexually attractive breed to me, can still be incredibly sexy in non-sexual ways.
I'd say that the main factors for any type of canine attraction would come down to intelligence, size and visual looks.
I've known/do know dogs which are hyper intelligent and I know a Collie+GSD mix that is THE smartest dog I've ever met. The level of awareness and control she has is mind blowing to me. The breeds I have experience with being around, ones that attract me most would be Collies, GSD, Springer Spaniels and Rottweilers (there are other smart breeds, but I haven't had much time around a lot of them to really know if they appeal strongly or not). I'd say the intelligence of a dog for me is a definite combination of sexual and romantic attraction. If I know a breed is very smart, then that is very sexually appealing because I know for a fact that he/she is fully able to comprehend mating, but knowing I'm with a breed that is extremely self aware is an amazing thing when you're around a very smart dog, and that simply adds to a definite romantic appeal.
In terms of size, it's pretty much any large breed is amazing. Huskies, GSD, Labradors or Weimaraners. The size definitely is more of a sexual attraction (for obvious reasons). But size tends to be linked to intelligence so I'd say there's a mix of a general attraction to go along wit them.
Visually, pretty much any breed that I can look at and immediately focus on (whether it's uniqueness of their coat, how cute they are, or simply the way their personality is in the way they interact with the world). Corgis, Collies, Burmese, Shibas or even Beagles are top picks. I haven't spent time with a lot of the best looking breeds, but they always stand out if I see them out in public. Most of the time, the visual aspect isn't even for a sexual or romantic attraction, they're simply beautiful or adorable breeds.
I doubt I'm the only one here that notices the variety of reasons to find some breeds sexy. A dog having a massive cock with a large knot or even an amazing looking cookie, is great, but often the attraction isn't only about sex. Like I said earlier, a breed can be sex for non-sexual reasons.