Thor is easily one of the most powerful beings in the Marvel Universe, and though he has a great deal of strength and power of his own, he's best known for lugging around a magical hammer called Mjolnir. Over the years, Mjolnir has been interpreted differently by different writers, and as this occurred, different versions of the hammer were introduced into Marvel's mythology.
Across the various Earths of the Marvel Multiverse, there's some kind of Thor, and while most have the basic hammer that debuted alongside the Asgardian in Journey Into Mystery #83, some stand out as completely different weapons.
To be fair, none of Thor's hammers could be considered "worst" on any list other than one like this article, which compares them to each other. Thor's weapons are significantly more powerful than most anything found throughout the Marvel Universe, but some are certainly better than others.
This list aims to identify the particular strengths, weaknesses, limitations, and enchantments that make one weapon better than another. Only comic book versions are considered for this list, and all told, there have been at least 13 different hammers used by one Thor or another in the Marvel Universe, which can easily be identified as being distinct from one another, and they're all laid out here from... let's call it least awesome to most awesome.