Mew, definitely. I never liked the Cerulean Cave itself even though I liked some of the weird stuff that you could catch in it. It was definitely my least favorite gen 1 cave, probably because my routine was to get to Mewtwo, use the Master Ball, and have to backtrack all the way. It seemed so petty to go through all that for 2 seconds worth of catching.
'Cuz it's better, stronger and has a wider movepool than mewtwo,
I mean it really is better than Mewtwo, despite Mewtwo looking better than Mew.
Mew is always better, and it's not manmade unlike Mewtwo.
Mew. It's simply adorable and very rare. Mewtwo you can get him in the cave and also in the remake. But for Mew, it's very rare and you can't get it unless by an event. Which makes it more rare than Mewtwo. And it can learn every single TM/HM.
See, I just loved Mewtwo. In the original games, he felt like the ultimate accomplishment when you caught him. He was the legendary you fought when you had already shown all of the other legendaries who was boss. He seemed like the epitome of accomplishment to me. And I could get him honestly, while I was watching all of the other kids gameshark their way to Mew.