They're both fun. Hetalia is best in small doses as it's a sketch show with a lot of repetition (for comedic effect). It doesn't look that dazzling visually and has very simple animation, but if you have even the slightest interest in history and/or cross-cultural jokes it has moments of greatness.
Slayers is a ridiculous fantasy spoof, and a lot of shows since have done the same kind of thing so it might feel a little dated if you've seen a lot of parodies (unfairly). The animation ranges from decent to terrible as it didn't have the highest budget at times, but the voice acting and music is first class. The high point for me was the second series (Slayers Next), though some of the movies are quite good too. The first few movies are standalone prequels without most of the TV series characters showing up, so you can watch one on its own if you want to try the series out. If you do that, I recommend looking at Slayers Great for pure comedy value.
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