I initially went with Mailbox and stuck with it even though updates have slowed to a trickle because I was hooked to the triage features. I then went to Spark because it is essentially a better Mailbox.
I can't use inbox because it doesn't have permission to access my work gmail account, and cloudmagic feels too slow and laggy for me.
I am trying Outlook now because it somehow seems to sport a more polished interface, and it has an iPad app (something which Spark still lacks). I have been wanting to move away from Mailbox for a while, but simply haven't found a better desktop mail app that supports push gmail, and as far as possible, I would like the same email client on all my Apple devices.
So no favourites yet.