I've always stuck to gmail.com (although I'd consider switching to inbox.com if I could get over my inertia).

I've never really had any serious issues with it. It did have a patch of slowness a year back but that seems to have been resolved.

I'm not sure how the offline capabilities have matured - as I'm very rarely away from a data connection.

I don't want to hijack the thread but what are your prime reasons for wanting an alternative interface?