The S&W N-frame .357 Magnum is one of the finest revolvers ever made, but it is heavy and a bit bulky - which makes it a pleasure to shoot. The K-frame Combat Magnum is a beautifully balanced .357 that shaved off a lot of weight and therefore was one that was carried more after its introduction. I don't know enough about the L-frame .357s to have a strong opinion (I only have a few of them). And, to me the new lightweight alloy .357s are un-shootable - way too light for the power of the caliber...
I am enamored with all of the N-Frames and have collected a bunch of each pinned and recessed model. I also have acquired several of those newer 8-shot 627s that are way fun to shoot! But I also went through a phase when I collected almost every variation of the pinned and recessed Combat Magnum... So with that in mind "Most sought after..." for me - Today it would be in this order:
1. Any factory engraved or factory two-toned N-Frame .357 Magnums from the 1950s. These are always my favorites and although I already have a bunch of them, I always seem to want more.
2. 5-screw 1950s .357s - I am specifically seeking one with a 5" barrel in as new condition with the box, tools and paperwork, but I would also be interested in a reasonably priced 5"er with tons of holster wear.
3. An 8.375" Transitional .357 Magnum and any 1950s 4-screw non-model marked .357 - blued in pristine condition (it would probably have "Julian Hatcher" engraved on the side...

or maybe it would measure at 8.5" and have some funky target grips...

)
4. An 8.375" Highway Patrolman - I have seen a photo or two of one, but never one live or one available for sale.
All the other ones are lower on my priority list as I have been lucky enough to wrangle one or two into my collection.
WITH ALL OF THE ABOVE SAID, I agree with Bill and think that for the most of us, the 3.5" N-framed 357 is the "most sought after" .357 magnum today, because of its: 1) beautiful finish, 2) classic lines, 3) shootability, 4) relative affordability, and 5) the crazy high rating on the "cool meter".