Chikorita's the one I chose. He's a personal favourite of mine and, as far as I'm concerned, a solid pick all-around. Hell, I bloody love this guy!
He doesn't have the hard-hitting early game punch that many starters have... and some team compositions need, but he does have early access to a string of useful support moves such as Poisonpowder and Synthesis. But it's really when you get past Corey that things really git gud. With the mining kit, you're pretty likely to get access to Light Clay immediately (and if you don't, you can always do a bit of nawty resetting until you DO find one), which allows you to access what I think Meganium's true role is: superb support for frail team comps. through extended Reflect and Light Screen.
I run a simple set of Synthesis/Petal Dance/Reflect/Light Screen on my Meganium, a set obtainable by level 40. I can't exaggerate how much of a lynchpin this guy is for my team, literally I could not have gotten through a single fight post-Shelly without hiding behind this guy!
Though don't get me wrong, he's got plenty of weaknesses. He never gets access to the better status moves, for one, and his best attack outside of a sun team is the unreliable Petal Dance. His 83 special attack stat is really pretty poor in terms of offense, especially on my Meganium who I stupidly accepted an ATK boosting nature on! Even his 80/100/100 tush loses its lustre later in the game, going from 'tanky' - and enough to set up the screens even against foes with super effective attacks - to just 'bulky'.
Well, at least Meganium actually is bulky, unlike the other 'bulky' pokemon on my team like Granbull - haha, they're the whole reason I need the screens in the first place, half of my offence is slow and lucky to take a neutral hit!
(Note I haven't gotten to facing Charlotte yet and I'm not looking forward to it.)