Gmail seems pretty good to me at least. The worst trouble I've had is a few seconds of delay to receive an email without a refresh. Usually my phone and tablet get it first.
Delivery is usually instantaneous for me, but on occasion I won't get email for 6+ hours. It's a lot easier for me to notice the delay when communicating with co-workers, as the sender often wants to know why I haven't responded to the mail he sent 4 hours ago.
