I've rarely had a phone successful in waking me up. I've had reasonably good luck with a Motorola Xoom tablet I still have that still works for this purpose. (The Xoom is a nice thick device that didn't slouch on component size, and can put out legit audio.)

But the most effective alarm clock I own is a good old-fashion quartz clock AM/FM radio. It's WAY louder than any phone or tablet can offer up, and having it is often more reliable as well, given that my phone sometimes needs a hard reboot after installing a security update and hence isn't booted when I wake up, or didn't align for charging correctly and ran out of battery while sitting on the charging pad.

On the other hand, the battery in the Xoom mitigates the risk of my alarm not going off due to a power outage, and it being smart mitigates the risk of being late due to a time change.