it's not easy to describe a 'best tactic' for man u. from all that i've played so far, (and i've played man u a lot), it seems as though they are designed to be inconsistent, no matter what tactic. though if it helps, i've had some success using a 4-3-2-1, or 4-2-3-1, and i've found that they can perform just as well with 3 at the back, using something like 3-4-2-1 or 3-5-2 | 
