Meganium wasn't my favorite, but I've warmed up to him.
I played GSC when I was a boy, so obviously I liked Feraligatr and Typhlosion because they were tough and cool. Meganium was the first time they made a starter that looked gentle and soft rather than big and strong. As an adult, I can appreciate more qualities than just coolness and toughness.
On top of this, he got the short end of the stick in terms of how good he was in his own game. Charmander had enough of a 'cool factor' and was pushed enough that he still was popular despite being really bad against the first two gyms in his game, but Chikorita didn't have that luxury.
I think dinosaurs are cool to this day, so it's neat that Meganium is the only dinosaur startermon to this day. Even if it's not from a fossil, it's clearly a dinosaur.
New Pokemon Snap gave Meganium a lot of shine, which was nice to see, and now he's finally getting a mega. I like this design based on the leak here
>>58174301, and I really hope his mega gets a dual typing (Grass/Fairy).