The simple fact is that there is no "best" Nature for any Pokemon. But there are better Natures, depending on the Pokemon and how you intend to use it. The first step is to look at the Pokemon's stat distribution of that Pokemon.
For Mightyena, we can see that it's physical Attack is it's highest stat. This is followed by it's Health, Defense, and Speed. These things indicate that Mightyena is probably best suited to more of a brawling role, giving and taking physical attacks.
This is further indicated by looking at it's movepool. Mightyena naturally has access to strong physical moves like Crunch and Sucker Punch, as well as good utility in the form of Taunt to block set-up moves from the opponent. It can also be bred with good coverage moves, dealing Super Effective damage to Pokemon types that would be Super Effective against it, like Poison Fang for Fairies and Bugs, as well as Play Rough for Fighting.
Looking at all these things together, we want a Nature that benefits Mightyena's physical Attack, or it's Speed. Depending on how you intend to use your Mightyena, a Nature increasing Defense to wall out other physical Pokemon. We can also see that Special Attack is not a relevant stat for most Mightyenas, so a Nature that reduces this stat is probably the best choice.
For these reasons, the "better" Natures for Migthyena are Adamant, Impish, or Jolly, depending on how exactly you intend to use the Mightyena.