Sparrow is easily the best mail client for iOS. It does everything you can think of and it does it beautifully. The one problem with iOS though is that you can't make other apps the default app like you can on Android. However if you're jailbroken there is at last a sweet jailbreak tweak which enables you to make it the default mail client, it also adds push notifications which aren't usually available with Sparrow.
Just download Sparrow+ from Cydia to enable push notifications and to make it the default mail client. It's completely free and is in the BigBoss repo which should already be on your iDevice.
Check out all our favourite Cydia tweaks - Cydia Tweaks and Hacks