Factory Ruger Mini-14 5/20's. Seriously.
I've bought every one I've found on the EE recently (which are only a few) and have paid dearly for them. Retail on them isn't cheap, but they're nowhere to be found.
Failing that, the newest generation of Tapco mags.
And one point I should note; all of the factory mags I have seem to have been pinned right at 5 rounds (tight) by the importer.
Like any mag, they need to be pinned to a length which only prevents loading 6 rounds to easily load, insert and feed.