Look, Undertale is not like most games, it's essentially an entirely different game in each of the 3 main "routes" making it highly replayable. It is also not a long game either, like 8 hours first time and considerably less for the other two (although there's a caveat). I basically did all 3 (4 because I screwed up one) in 15 hours.
I don't agree with people saying play pacifist first time, it doesn't make up for the neutral playthrough. You'll need a guide to get it because you have to talk to people and do things at certain times to get the pacifist ending, and characters behave considerably differently on neutral. There's a lot more to the playthrough than just the different ending. And pacifist is only really as impactful as it is after you've already experienced the neutral playthrough.
You can play it once and it's worthwhile doing so but you'd be missing out. After my first playthrough I just thought the game was decent, but I could see there was more to it. Kept going into 2nd and third playthrough and it eventually became one of my favourites of all time. It's in those later playthroughs where the game really blossoms, and it's only so good because you've experienced it as a whole. Every playthrough builds on the last.
Seriously though, just play it blind however you want, and if you're intrigued at the end keep going is my recommendation. I'm not the type to replay or care about alternate endings either, but Undertale is different and it will subvert your expectations.
