If you're new to the Hetalia fandom...
The webcomic: :)
The manga: :)
The anime: Heavily based off the webcomic and manga. Nothing to worry about, just brace yourself for the LSD trip-esque endings.
Nekotalia: The nations have cat counterparts. It's adorable. As a bonus, they have Nekotalia segments during a few of the anime episodes.
Nyotalia: The nations have genderbent counterparts. If you're curious to see what they're like in Nyotalia, they have an episode that you can watch.
2P Hetalia: This was a concept that Himaruya invented, but although the original idea was a simple colour palette swap, the fandom developed it into more of a "what if" universe if the characters "dark sides" (or opposite personalities for that matter) became their dominant personality. Have fun.
Episode 23.5: If you have any, ANY respect for Italy, do NOT watch/listen to this. I swear to God, if you watch it, you'll regret it for the rest of your life. There's a reason why this is a CreepyPasta.
HetaOni: NO. DO NOT. STAY AWAY FROM THIS. TOO MUCH TRAGIC. TOO MUCH NOPE.
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And lets not even get started on Gutters or George DeValier.