Which LMS do you use and why?

I can't recommend it, sadly.

Learn.com was weak in a few areas, but overall a solid tool in its time with a fantastic support team.

The Taleo acquisition relegated the Learn component to a back-seat position, so development was no longer quite as robust and the support took a hit.

Since Oracle acquired the product, it has suffered significantly. A large portion of the development is now related to fixing bugs and modifying the system to match Oracle design standards. That makes it difficult for the system's development to keep pace with the market-leaders. Add to that the support issues that plague the Oracle version of the product (think >18 months to correct some issues caused by their own 'updates'), and it paints an unfortunate picture.

The base system is still solid (if a little dated) and the tech team is knowledgable, but unless you've got some serious contracting leverage and can build in penalties for service issues, I'd set my sights elsewhere (and build acquisition insurance into that contract, as well).