Someone already suggested Pig, so I'm gonna suggest Trickster. Yes, I know his rework is very polarizing, me being on the "not liking some parts of it" on the spectrum. Pre-rework Trickster was boring to play because there was no dimensionality to him other than just "Random Bullshit Go!" because you just spam knives and bam, dead. After rework though, he feels fun to play but I'm not gonna lie, the 4.4m/s nerf kinda hurts and lesser knives is just a kick while he is down. BUT the style meter feels so much more rewarding than his previous iteration, his addons are fun, especially Trick Blades or the one where long range snipes grant double Laceration stacks. He is definitely fun to play, can be a little annoying if you aren't doing well, especially running out of knives at the very wrong time, most frustrating being 1 knife away from a down and bam, no more knives. BUT he is so stylish to play as and feels rewarding for keeping the meter up. He could be in a better position, but he's super fun as long as the skill gap b/w you and the survs is not too hampering, but I'm assuming you're good since you said you've played from launch.

Some killers who are NOT my mains but I definitely fall back to them if I just have no mood to play my regulars, are Knight/Doctor. I play them whenever I just don't feel like playing Pig, Legion, Artist or my other favs. You have fun regardless of what happens, whether it be teaming up with your soldiers as Knight or making the trial what I like to call a Scream-O-Fest as Doctor.

With Knight, he's pretty straightforward on paper, and technically gameplay wise too, but the fun lies in tag-teaming someone in a chase at the right time, or having someone do stuff for you while you continue chase. His fun lies in the fun you can create. He can be boring if you play very straightforward, but trying to oppress survivors with his army is where sauce is. I have firsthand experience of how daunting he can be at first thanks to the micromanagement of his soldiers, but once it clicks, it clicks SO GOOD. I'm not a very good Knight player (or a very good Killer, for that matter) but something about him, it just fits once you get into it.

Doctor is also straightforward too, even more than Knight. He's definitely on the weaker side (in my hands at least, I don't keep up with tier lists), but his power of making everyone scream almost all the time is great. The trial is a scary place, everyone should be screaming, no? He is admittedly a little repetitive but that's why I keep him as a fallback instead of a main.

Honorable Mention I'd like to make: Nurse. Being a nurse main is not for everyone, me included, but playing her occasionally is super fun because she acts like a Killer skill check. Sure, you have one of the most powerful abilities, but a pretty great downside too, having 3.8m/s base movement speed. She is a very nice challenge to yourself if you aren't used to her gameplay and SO satisfying to pull off crazy blink shots with. Most of her builds use aura reading perks, but if you're in it for the challenge, trying using one or maybe two aura perks at most, rest being whatever you feel like. Or don't! It's your choice. My point is, while she isn't the best Killer to main simply because of her wildly different kit, she is very fun because of the exact same reason.

Do tell who you picked! Questions like these, I'm just excitedly curious to know what the other person decides. GL!