He certainly is, and I might be the only one to disagree with you so far but also see Conor's achievements.
He's up there, he KOed Aldo in 13 seconds which is impressive, Dustin KO was great and KOing Mendes at the time really meant something.
He is absolutely up there, however Overeem has power everywhere - he's KOed not only guys with insane chins but guys with high level striking in K1 which is much harder to do than KO a wrestler.
Long story short, yes Conor is impressive - maybe top 5 hardest hitters but Overeem is slightly higher (P4P) due to who he has KOed and how he has KOed them.
Overall - Hendo would get my vote, KOing people with one punch across four divisions.