To be completely honest, none of them; I like the grind, and getting free anything would feel like cheating to me. If I had to pick one disregarding that though, mana (although I'm currently at mine VI and know nothing about the last 3 you mentioned )
Definitely not #2 because that would quickly get lost on rebirth.
Neither would I want 1 million mana as I would basically of have beaten the game and then it loses it's fun.
I would probably take the magma farm (If automatic) as it is a constant stream of money.