There are literally too many comic book characters to list who could, in theory, be killed with a bullet to the head. There are a lot of characters without any super powers at all, for example, who are “normal” humans who just have training and/or gadgets.
For the most powerful characters that could be killed that simply, just with some planning, there are quite a few powerful magic users and reality manipulators who are otherwise normal humans who could be killed by a bullet they just didn’t see coming. Off the top of my head, in no particular order:
Dr. Strange
Zatanna
Scarlet Witch
Franklin Richards
Shaman
Dr. Fate
Arion of Atlantis
Molecule Man
There are also a number of characters who transform into powerful forms, but have normal humans as alter-egos (again, just off the top of my head, in no particular order):
The Hulk (Bruce Banner)
Thor - sometimes (Dr. Donald Blake; I think in more recent continuities, he usually no longer has a mortal form)
Other wielders of Mjolnir or Stormbreaker: Beta Ray Bill (powerful cyborg alien even without the hammer, but probably could be killed by a bullet to the head), Thunderstrike (Eric Masterson), and others, depending on the continuity
The whole Marvel Family: Captain Marvel (Billy Batson), Captain Marvel, Jr. (Freddie Freeman), Mary Marvel (Mary Batson), Black Adam (Teth-Adam), and others, depending on the continuity
Since Captain Marvel is tag-lined, “The World’s Mightiest Mortal”, but has a human alter ego that could be killed by a bullet, he’s arguably the most powerful character that could be killed by this tactic, but several of the magic users/reality manipulators above are often portrayed in ways that make them seem significantly more powerful than the Big Red Cheese.