Jef Lippiatt Owner at Startup Chucktown
I think it really depends on your needs and comfort level with software. YouTube has a built in "Creator Studio" it may not be as robust as some apps, but obviously it integrates well with YouTube.
I know many people that use Adobe After Effects, but if don't already own a copy or are not familiar with it, the pricing and learning curve may overwhelm you.
I'm not really aware of any mobile apps specifically designed for creating explainer videos, but I'd be happy to hear about them from others.