The best manual focus screen for the 5D is the EeS model - lots of in-focus 'pop'. Unfortunately it is also darker than stock. If using AF and just want a brighter screen - may want to look at after-market like BrightScreen or Katzeye.
NOTE: This varies by individual 5D body, but changing the focus screen in a 5D from the one originally shipped with it from the factory, EVEN to OEM Canon screens like the Ee-S may require you (or Canon) to re-shim for accurate MANUAL focus.
Unlike the 1 series bodies, the 5D (and one assumes the MkII as is same body) suffers from manufacturing variances in the prism housing and the factory shims each body to the stock screen for 'close enough' manual focus.
ANY change to another screen, regardless of who makes it, including Canon, could require a re-shim to cure resulting back/front focus issues - using manual lenses.
Again, like the famous 5D mirror clearance issue with alternate lenses, it varies by sample. I ran a 5D (since replaced with a 1DS2) and a 1Ds Mk1 together for some time. While my 5D would take damn near any Leica/Zeiss/CV/Nikon lens I tossed on it (some of which were supposed to be problematic), I had to re-shim and re-test focus every time I switched screens (settling eventually on the Ee-S) where as the others could switch screens with little if any issues - while hanging on the lenses mine cleared.
The Camera can be sent to Canon with the (Canon only) non-stock screen for calibration, but you'd do a more accurate job yourself as Canon tolerances for screen shimming assume you'll be using AF and all the screen is for is for framing and for indication to you that AF has been achieved.
Shims for the 5D can be bought from Canon parts or some use narrow strips of metallic duct tape (it's apparently 0.001" thick) or scotch tape on the edges of the screen.