The spider.
Story: It was my very first night playing MC towards the tail end of Alpha. I dug a small hole into the side of a mountain to shelter for the night, but left a 2 block wide window for me to look out.
I was watching all the mobs spawn across the surface feeling a bit nervous, until I watch as a spider (first time seeing one) slowly rise up to meet my eyes in the window. Because the window was 2 blocks wide, it crawled into my hole killing me.
It's my most vivid early MC memory that's stuck with me. So to me, the spiderbro is best bro.
Just looked it up and apparently enemy mobs have been able to climb ladders for over 10 years, but their AI pathing doesn't try to go up ladders so its uncommon.
I never knew they could either