Ok. Though I could name a few Stands I'd want in general, adding those specifics to it actually puts more thought into it. So many Stands instantly become impractical. A lot of Stands would only be nice to have if you'd either use them to play pranks or more 'malicious' reasons. Others are only practical if you have a specific career. Basically 80% of all Stands are instantly dropped, with another 20% being subjective in use and finally only 5% that could be universally useful. So, after going over and giving myself quick reminders of how Stands work and what they're capable of and brainstorming how the various Stands could potentially affect my life, here's what I've got.
It's honestly a tie between Hey Ya! and Mandom. Weird, I know, but let me explain.
With Mandom you can rewind time by 6 seconds. Through it'd be tricky to get it down pat and properly use it and I personally feel it isn't as good as Araki thinks, it's still useful. It's still good to have. If I'm careful, I can have it at the ready before I do something I'm not sure about. The moment I see signs it was a bad idea for up to 6-9s (or so) ago, I can quickly rewind. I say 6-9s or so because I'm sure there are some situations where, even if I'm in the middle of doing it, I could turn back. You don't have to rewind all the way to the beginning, just to the point you can correct your mistake. However, this is if I'm prepared and am keeping track of time. If I don't keep track of time, then I won't know how many seconds it's been and as such won't be able to tell if I'm still safe or not. And, if something happens spontaneously, I don't think I'd have the reaction time to use Mandom. I mean, six seconds might be enough time to summon it and activate it, but I suppose that would depend on the situation. After all, the clock hand needs to be physically moved or I'd need a wristwatch like Ringo to activate it. That is, unless, I somehow don't need to, unlike Ringo. If I can consciously activate Mandom's time rewind without a mental switch or to physically move the hand, then I'd be set. Just give me 1-3s to react and I'll be rewinding time. Basically, if I practice with it and eventually get to the point where I trust myself in my ability to properly use it, I can more confidentially approach various situations in day-to-day life. The only issues then would be scenarios in-which I know the point of potential no-return will be well past 6s, meaning before I'll even realize I've made a mistake, I can't just rewind time because at that point I still have to commit to the action. With that in mind, then there's situations where I'd have to gamble whether not I'd want to risk it. Where I'm not sure if I should keep going or turn back before it's too late or even risk at all in the first place. As someone who over analyzes things and - because of that - finds it difficult to make decisions, I'd probably waste my time contemplating.
Then there's Hey Ya!, a Stand I know a lot of people joking say they wished they had, but I'm serious. Though it isn't practical in the same way as Mandom, it'd probably do wonders for someone's mental health. Having someone always over my shoulder encouraging me and telling me what it is I have to do would genuinely be wonderful. If I had Hey Ya! cheering me on, I'd probably have more drive to do literally anything, have confidence in literally anything I do. Though it doesn't actually affect luck, thus leading me to imagine scenarios in-which it makes me overconfident and, thus, failing miserably (maybe even dying), it's still a Stand that I'd be more than glad to receive.
There we have it. I'm sure OP meant to pick a singular Stand, but as I said- I'm indecisive. Whichever one it'd be, I'd except either way.
Another Stand that'd probably would suit me just about as well as Mandom or Hey Ya! would be the Stand that's commonly hailed as the best Stand of all time, Heaven's Door. The reason why I don't choose it is because it's both a basic answer and isn't quite suited for my life, specifically. It's just so versatile that it'd probably fit. But ultimately I'm just not too sure what it's capable of or how I'd even use it. If Rohan can write in himself just as much as other individuals, then that'd be great. It's just that, as far as I can remember (seeing as it's been a bit sense I last watched DiU), Rohan has never really done it in the main story. He has done it
in one of the OVAs, but I haven't seen the OVA yet, so I'm not too sure as to how the command is eventually undone. As such, I'll refrain from outright saying what it is and isn't capable of, but just know I'm too unsure to confidently say I'd want it (in that aspect). If I can write into myself "You have an IQ of 200" or "You will do X" or "You possess another Stand", I'd absolutely choose this. There's also the matter of can I write "You possess a Stand" into another person, as well? If I can, either way, I'd potentially have an infinite arsenal of Stands and a means to see what Stands other people would possess (which would genuinely intrique me). If that's the case, then I could probably go all the way into the details of the Stand and it's ability. Though, even if that isn't the case, Heaven's Door has an A in Potential, meaning it's actually more of a possibility than you'd think. So, now that I've gone over it
a bit more, I'm slightly regretting my decisions. But as I said, Mandom and Hey Ya! personally suit me more whilst Heaven's Door is overall versatile and a bit of a wildcard, all on top of being a basic option.
I know Pearl Jam, Gold Experience, D4C, etc. would be good as well, but for Pearl Jam you'd need good resources to properly use it. Also, I'd have to get someone to cook the foods. Or maybe pick up cooking. Just don't expect 5-star meals, ok? Gold Experience- cool to have, but I don't think I'm clever enough to use it nor know enough about plants and animals to properly use it. D4C was my first thought, but, from my understanding none of the alternates of Funny Valentine possess Stands of their own. So, if we are to assume that the other mes don't possess Stands and expand the prerequisite of "you're the only Stand user in the world" to "in all the multiverse", then the only interest I'd have in it's ability would be to see the various dimensions and what they hold. ("I'm an extra-dimensional tourist, lol.")
Well, that's it. I think I've already said more than enough. Thanks for reading. :)