I played HEAVILY with VOIP on the Android to see if I could use it in an on-the-go Podcasting capacity, so I'll share my experience:

Sipphone is... not good, in my experience. Very tragic product. That being said, there are far worse tragedies out there for VOIP on Android.

As far as the best goes, I had the best luck with using Linphone on Android, and it seemed to work the cleanest of the whole bunch.

All of that means nothing if the conversation is about Skype, though. Since Skype is a closed-network and NOT a standard SIP protocol VOIP system, none of this software is going to matter to you.

If you want Skype on Android, you're likely going to need to be on Verizon. If you don't necessarily want Skype in particular, just ANY VOIP will do, then you have a plethora of SIP client apps available (and a few other less used VOIP protocols).