Lao-Shan, Shen Gaoren, Jhen Mohran and Ceadeus were definitely memorable for me. Sure, first two were pretty dull and Gaoren only did a bit more than Lao-Shan, but holding back two monsters that were pretty much moving fortresses and actually killing them with weapons that were pretty much just toothpicks and lollipops to them was very satisfying. Jhen was very fun. Something about using the ballistae, cannons and dragonators of the ship was really cool. Ceadeus gave me a lot of problems. I wasn't really that good at underwater combat and it pretty much just rammed and shot me consecutively, pretty much stunlocking me. But its battle themes and arena were great and I enjoyed underwater combat despite being bad at it. The very best part, though, is when Proof of a Hero/the MHDos theme for Gaoren plays in the finale or after the dragonator scores a hit. Excluding Ceadeus, but that would've ruined the tone of the fight for me.
I remember first playing MHFU and being really bad at it. I had a hard time with even Giadrome and Yian Kut-Ku. I was strictly blademaster until I watched Vodgoblin play it and convinced me to use gunner weapons too. That really improved my experience playing the game and I managed to make it to HR solo. I got better overtime but I guess not good enough to get to G-Rank. Also the game where I first got into longsword and I've mained it since then.
A lot of others too, like my first multiplayer hunt in MHGen where me and my party tore through Gore Magala, first getting MH3U and being amazed by the underwater environment, all the rage while playing, all the mishaps, the fun and the feeling of satisfaction after crafting gear.