>>2 I would recommend life steal as it will allow you to survive longer as long as you hit enemies.

>>6 This is the undeniable truth but I just want to get the obsidian chests. I also finished the game so I don't really need to grind that hard anymore.

>>5 That's right but you'll receive fewer rewards if you don't defeat all the mobs in a level.

>>3 If you want to speed run a level or just want to get the reward fast, you should go for movement speed buffs.

>>3 Same. If you got the medallion, the mobs you can get also get the bonus damage or lifesteal. also works on pets.

>>2 Personally I would not go for movement speed aura. Life steal or damage auras both work just fine depending on your play style and current enchantments. Unless you are doing a soul build however you should probably go with the damage aura because of the other bonuses the armor gives you.

Hello, I'm planning to play this game by myself. For single-player, which aura is the best to have?

It really is better to have friends while playing the game. You can only get multiple aura buffs when you play with them.