SQL Server 2005 » 2012
From a DB-Migration point-of-view, restoring a DB from SQL Server 2005 on a 2012 server is not at all a painful process. It's what surrounds it is where the pain starts: SSIS packages and the like. What tripped us up was the .NET application that had been written for and ran on Windows Server 2003. That wouldn't run on Windows Server 2013 and the migration was cancelled.
I will echo the sentiment about overkill — if you have a small DB (under 10GB), then the Express Edition can be made to work. It only supports one processor (possibly only one core — I'd have to check that up) and has no SQL Server Agent, but the Scheduler in Windows can be used to run sqlcmd. And if your DB is not that much over 10GB, then a careful refactoring of datatypes (DATE instead of DATETIME for dates, VARCHAR instead of NVARCHAR when you don't need Unicode, SMALLINT instead of INT when your field only has a couple of thousand varying entries etc) and a regular archiving DB policy, so that seldomly-used, older data can be offloaded to the archive DB and thus bringing the main DB down under 10GB.