Meganium. I started playing late (with HeartGold - I was eleven and denounced Pokemon as being for babies and for boys when it came out here and ignored it for over a decade, and yes, I sorely regret it), and this guy was my first ever starter. I knew basically nothing about the series other than the fact that my friends loved it, so I was basically... exploring Johto for the first time, discovering this whole world, and my Meganium was my faithful partner the entire time.
There are other Pokemon that are better (...a lot of other Pokemon that are better), there are Pokemon I've used more, there are Pokemon whose designs I like better, but Meganium was my first ever starter, and I just can't... forget that.
Runners-up: Umbreon (also a complete bro in HeartGold, and adorable as hell - and reminds me of my old cat), Absol (loved the backstory and design, finally got to use one in W2 and adored it), Sigilyph (used one in both White and W2, saved my team's ass in multiple occasions, amazing design and I really like the typing), and Lucario (caught a Riolu in W2 to deal with Cheren's gym, ended up using him in Roxie's, taught him Ice Punch and used him in Skyla's gym along with my Ampharos, solo'd and OHKO'd most of Opelucid Gym, and was the almighty dragonslayer for all of the plotty game stuff, now I'm doing postgame stuff and he's by far my strongest member of my team, at level 88).
