The par 5's at The Ocean course are not for me at least the most memorable reasons why I love the course and Harbor Town has a pretty mundane set.
Regarding the Ocean Course, I agree that the par 5s are not as instantly memorable as some of the other holes (12-14 and 17, for instance), but I think they are among the strongest holes on the course.
I love the angles of play on the 2nd. It is one of the few holes where I found myself having to think about the placement of the tee shot even if I had no plan to go for the green in two.
I love the green site of the 11th. To me, it is the prettiest and most visually intimidating on the course. I also found the relatively small green much easier to approach with a full wedge than a shorter pitch, especially from a bad angle -- another feature I admire greatly in par 5 holes, since it really makes you think before you blast a second shot.
The 16th is not one of my all-time favorite holes, but I think it is an excellent complement (visually and in terms of shot values) to 17 and 18. I also really like the massive bunker right of the 16th green: it provides an appropriate degree of penalty (somewhere between 1/2 and 3/4 of a shot, depending on where in it you go) to make the second shot decision a close call in a lot of circumstances.
I didn't find the 7th that exciting, but I do think it provides a good contrast to the other three holes and possibly the most interesting non-par-3 tee shot on the course (the others I'd consider are 2 and 13).
I've only the played the course four times, so my opinions might be influenced substantially by the tee setup / wind conditions when I played.
Regarding Harbour Town, I agree that the par 5s are the weakest set of holes on the course. I also think the par 5s at Teeth of the Dog are not that great. Overall, I think Dye has built many, many excellent par 5s but that the average par 5 on a Dye course is not much better than one would expect given the quality of the his other holes.
As to the original question -- who builds the best par 5 holes -- how about Stanley Thompson? Though many of his par 5s have been weakened by technology (see most of the par 5s at Banff Springs, for example), I find them creative and challenging without being reliant on water as a hazard. I think the set at Highland Links is truly excellent (especially 7 and 16, two of my favorite holes anywhere) and the set as Jasper, while very short now, is also fantastic. Judged at the time they were built (since we can hardly blame Stanley for subsequent advances in tehcnology), I think Thompson's par 5s are without parallel.
Edit: sorry, missed the word "modern" in the first post. But I think this raises an interesting question: do modern architects build better par 5s? Or has technology weakened most old par 5s to the point that we can't really evaluate them meaningfully anymore?