There's no third level on the Stretford End. It's a two tiered end. Tier one is broken off into Lower and Upper sections. If you look at the North Stand, tier 3 of that listed as N40X. It's like that for the same reason as the Stretford, because Tier 1 is split into Tier 1 Lower and Tier 1 Upper (Lower is where UNITED is on the seats, Upper is where MANCHESTER is on the seats).
From all reports this is what I would say:
Stretford End (mainly tier2, since half tier 1 is family seating)>>>> East Stand >>>> North Stand >>>> South Stand.
East Stand is great, the old Scoreboard End/K-Stand. There's still a decent voice from the fans there, plus if you're EL133, you're right by the away support, which can always be fun (as long as we're winning
)