This is a very close one, but I'm gonna pick Spiritomb. It was always that mysterious pokemon that no one had, and it has an awesome design. Cynthia also used a Spiritomb, and that was also really cool to me.
While Sableye's Prankster ability makes it arguably more useful in competitive battling, I view a Pokemon to be most wholesome as a design when it has usable stats, something which Sableye does not have. Without Prankster, Sableye is worthless, while Spiritomb is not defined by its ability so it gets my vote.
Not only is it cool, it's actually obtainable in game without having a) 100 friends that play Pokemon all accessible in real life b) Wi-Fi for trading. It looks creepy and cute and badass all at once somehow. It's got a great ability in Prankster and is an absolute blast to use in competitive battling because of how uniquely it plays.
Spiritomb, on the other hand, is a Pokemon that is nigh impossible to get on your own, looks pretty dumb, and is boring aside from its lack of weaknesses, which Sableye already did anyway.
Sableye. I used one in Ruby, and it was useful enough to convince me to use it again in Emerald. Then fifth gen rolled around, and it became useful competitively too. Plus, it has an awesome design, and it eats gems. IT EATS GEMS.
Sableye. Everything about it screams mischievous, and obtaining Prankster by Gen V just made it stick to its theme more. Also the first time I saw it, I felt a slight reference to Kamen Rider so that's a definite plus for me. Compared with a swirly blob that has arguably the most annoying catching method (right up there with R/S/E Feebas), Sableye beats it quite handily.
Okay okay we've had three bad rounds on the trot where you guys have voted for the wrong pokemon. No more, I say. Spiritomb is by far the better of these two pokemon. I'm not saying that Sableye is bad, and it does look genuinely creepy, but I've actually used Spiritomb in game and came up with a nifty CM set with it. I've never battled with a Sableye and just find them annoying to face, as well as the fact that I hate every pokemon card of the little guy.
Sableye, since I have memories of it in Emerald (won the Master-rank Smart contests in Emerald with it), it looks sweet, and it has Prankster, which boosted it from 'absolute crap' to '*insert trololol song here*'.
Sableye because he has a cool ass design and isn't tedious as fuck to catch. Plus, Prankster is nice, and Sableye is probably the best Prankster user outside of Thundurus and potentially Tornadus. Plus, I'm a gen 3 babby, so nostalgia and whatnot. :P
Spiritomb is a really cool mon, though. Pursuit / Sucker Punch mindgames are always fun to play! Anyway, it also has a far weirder design and far cooler connotations (Sableye is some gem demon... Spiritomb is literally the living embodiment of sin ffs).
Magistrum said a good portion of what I was going to say. I'll just add that I like Sableye's design more than Spiritomb's - I mean, Spiritomb is more obviously sinister, which is a point in its favor, but Sableye hits a nice balance between cute and creepy. Those huge crystalline eyes, the manic grin, and the hanging claws just look so excellent. (The fact that Sableye eats gems is also pretty cool.) To top it off, Prankster Sableye is one of the most effective trollmons in competitive play, and I love annoying my opponents.
Never has the number 108 meant so much... 108 kg, 108 cm, #108 on the Sinnoh Dex, Base 108 Defence & Sp. Defence, referring to themselves as plurals in the PMD games "We are 108 spirits!"... And ridiculously difficult to obtain.
Sableye? Ehhh, not even diamonds can help it from being more cooler than something so numerically inclined, lol.