Bah, from the thread title, I was hoping this would be a "just for fun, if you could do any game possible, which would you pick?" y'know, to see people's personalities and stuff, but number 3 ruins that, because yeah, I would be someone who would pick something obscure if I were to get into speedrunning because that's legitimately what I would have the most interest in running to begin with (and have the bonus of hopefully introducing people to some cool stuff they have no idea about because while they would be obscure games, they wouldn't be terrible ones).
Making it some serious "deal with the devil" type thing ruins the fun of this for me a bit, because yeah, some people would pick obscure games to run if that were their choice... but that's just fine if that's their interest of course, and others would pick popular stuff, and it would just be cool to see what type of stuff people would pick either way.
So if that restriction doesn't exist, just for example purposes, I'd probably pick something like one of the Detective Conan Game Boy games. Like either
The Mechanical Temple Murder Case or
Legend of the Treasure of Strange Rock Island (which I'm kinda surprised have entries on speedrun.com period; granted, they each only have one run, but I wasn't even expecting their to be a page for either of these games), probably the former. And part of the reason for that would yeah, they're very obscure games, especially in the US, and I feel those would be part of their appeal if I were to speedrun them due to simultaneously exposing people to overlooked games and giving a good tastes for my preferences and tastes in games at the same time.
But why those particular ones to accomplish that? Well, they're basically mystery/adventure games that just advance by talking to people and getting all the information, evidence, and testimony you need to put the pieces of the murder cases of the game together. So that is to say, no combat or anything like that. A speedrun of a game like that would pretty much purely be focused exclusively on efficient routing (as in, taking care of stuff in the fastest order possible), efficient movement in general (as in, getting good at moving Conan and the other characters in the games in general and not just doing stuff in the best order but also just wasting as few steps as possible to do it and avoiding bonking into walls/walking passed NPCs and evidence as you're trying to go as fast as possible), and stuff like that.
It would be pure execution of knowledge of the game and movement, and if I were to get into speedrunning, that's probably the type of thing that would appeal to me most personally. As that might not sound exciting to others, and that's fine because a.) I'd be the one watching it, not them, they'd just be watching :P and b.) it's just like the laser-focus on one particular skill that would really appeal to me in terms of speedrunning. Like, picking a game like say Ocarina of Time for instance, there's so many different things you have to get good at in a game like that, like not just movement tech and stuff but learn all the glitches and stuff you need in the any% route and even if you're doing a glitchless category, you still have to not only become a pro at movement but also become a pro at using all your items and stuff and make sure that like you have all the strats for killing the bosses ASAP down-pat, and stuff like that. Or perhaps a better example is something like Pokémon where again, movement is absolutely huge in Pokémon game speedruns, believe it or not. But so is ability the think on the fly and make very quick routing decisions when say an unexpected critical hit or miss happens (or alternatively, the AI on an enemy trainer decides to use a move it doesn't normally use or hits something it rarely does and you just have to make do with that situation and figure out what to do really quick as stuff like that can easily up quick in terms not only of the damage you're taking, but also PP management in your moves, money management in terms of necessary healing item use to deal with those situations and just making due with limited resources due to going as fast as possible and even Pokémon Center visits using a lot of time so if something unexpected happens things can get real messy fast and dealing with that in a speedrun setting is not easy at all and is not only something I'm not exactly good at but very likely wouldn't have much fun with), and you'd need to get extremely good at situations like that to move up the leaderboards on games like that as well.
And just for me personally, being able to give my all to one particular aspect of a game and just laser-focus on it is something that sounds like a much more enjoyable experience to see just how far I can push that one particular skill and how good at that particular type of skill I can get without anything else to distract me/split my attention. And mystery-adventure games/point-and-click adjacent-esque stuff like those Detective Conan games I mentioned would seem to be a great way of doing that while exposing people to some cool stuff that they probably don't know about (since they were never released in the US and with their age most people probably don't know about those particular games regardless, and yeah).
As for why Mechanical Temple Murder Case over Strange Rock Island? Well, mainly because the map of the game is smaller than Strange Rock Island and since I've never really done anything of the sort before, I'd want to start small and then work my way up. Also, playing them casually recently, my feeling was that Mechanical Temple Murder Case is just a shorter game in general, but looking at the times at speedrun.com, both are about the same time which is pretty interesting to me. But either way, with either of these games, another part of the appeal for either of them was definitely that they felt like they would be short and quick games to speedrun that wouldn't have taken more than a few hours at most, as there's no way I could do like 8+ hour speedruns. About 3 or 4 would definitely be my limit, and either of those games easily fall into that.
But in any case, playing more along with the thread's rules... I'm not really sure, honestly, because most of the really popular stuff isn't really something I'd ever have any interest in speedrunning myself and I'd pretty much be doomed. xD Like, the closet thing that occurs to me right now is something like
Hamtaro: Ham-Ham Heartbreak, which, according to speedrun.com only has 26 times, so it still doesn't really count under those rules, but... close enough hopefully?
But as for why Ham-Ham Heartbreak? Honestly, a lot of the same reasons as the Detective Conan games I mentioned: that because of the type of game it is, it would be more focused on movement than anything, and it's a very short game even casually if you know what you're doing, which both appeal to me for reasons I've mentioned already. Except unlike the Detective Conan games it's obviously nowhere near as obscure. But on the other hand, it's a completely adorable and super heartwarming game that I adore (and that more people should know about regardless), so that's more then fine.
Unfortunately, for exactly the reasons I like it (it being an adventure game where a speedrun would almost certainly exclusively be focused on stuff like routing and movement tech), even with the only 26 entries I seed on speedrun.com, while I haven't actually watched any of them, I'm going to go ahead and guess that the run is pretty well-refined and I'd be pretty much doomed if I were to try it. xD
But nonetheless, that's where, hypothetically speaking, my interest would be if I were to get into speedrunning in any capacity, so that's the type of stuff I would go with myself, probably.
(And also, I'm sure this is a much more detailed reply than you really expected from anyone. But this is a topic I've actually thought about a lot, not that I have any plans to do so because even if I had the opportunity I know'd I'd just keep using my time to do other things instead but it's just something I have fun thinking about sometimes all-the-same because I do watch a lot of Twitch and stuff, so it's just fun to get my thoughts I've been having for a bit anywhere actually out somewhere on paper, so to speak).