What tubes are best for a Mesa Boogie Single Rectifier?

I prefer groove tubes You can go with either 6L6 or the ELs but make sure that you get them biased correctly. You can't just swap the tubes yourself without getting them biased. This needs to be done every time you change the tubes. If the amp is brand new than the tubes should be also. If the amp is old than find out if the tubes have ever been replaced. If they have fairly recently then try the tubes that are in it. You may decide to keep the 6L6s if you are looking for a more of a heavy metal tone