Charizard, because of the sheer amount of hate he's gotten from both the community because of being "overrated" and the competitive scene because of Stealth Rock. Be lucky that he along with Gyarados isn't weak to Fairies like the other dragons.
Wait....are you suggesting that it's underated for being overated?...I don't think that makes much sense.
It is possible for a Pokemon to be considered as underrated for being called "overrated" in a negative term, because it means they're underestimating its capabilities for its exaggerated capabilities brought up by non-competitive players and anime fans.
No no no, that's not how those two words work. To be overrated in our context, the subject must be so well received and liked that it's supporters refuse to see or acknowledge its faults.To be underrated, the subject must be widely unpopular and its redeeming abilities dismissed because of said unpopularity.
And based on the fact that everyone who grew up when Gens I & II just came out knows Charizard as one of their favorites or in the very least a complete beast, we can determine that it's not underrated...you'd have to be silly to honestly think that.