Depends on who you've masteried so far. I don't know where you're at progress wise so I'll just share the whole thing... 

Your starter gets his/her masteries first with gems, then the next champ you use mainly, so perhaps an Ultimate Deathknight or Deliana. After that, typically we recommend people push up to Mino 15 and farm the masteries themselves using those two champs as the backbone of the Mino farming team. But buying a third set to make that easier is probably okay too.

As far as masteries for FroBan or a Felhound/Bellower type, you'll be getting masteries on both, so which one you start with is up to you. Typically a campaign farmer helps with progression faster, but a FroBan will help you push up in Dragon if you're struggling there.

When you say "speed farmer", do you mean campaign farmer or a faster farmer for dungeons like Dragon?  If it's the former, then Frozen Banshee won't help.  If it's the latter, then she will

I echo Quinn's comments above in that your first priority is a solid campaign farmer which is usually your starter champion, and this is where your first 800 gems should really go

I meant campaign farmer, yes. Thank you for your advices, guys. Then I will use gems on a campaign farmer🙏