AP/Str are never, ever better for tanking that hit (below cap), or block value. All they do is make threat, and they make less threat that hit rating an block value.
Agi is pretty darn good, since it is basically crit + dodge. (Crit is like a bad version of hit). 2 agi is better than 1 hit + 1 dodge. Its awesome for situations where both threat and avoidance are good.
For heroics, once one already has reasonable effective health and uncrittable, I'd say (all comparisons assume equal itemization points worth of those stats)
Block Value > Armor > Agi > Hit > Defense/Dodge/Parry > Stam > Crit > Str> AP > Block Rating
(if one has trouble surviving, avoidance moves up the list in importance)
I'd put defense > dodge > parry in that order, though they are nearly identical in value. Defense gives 4% less avoidance than dodge, however some of it is parry (which gives a bit of threat, especially against many mobs), some of it is miss (which works on mobs behind you, which can happen in multimob tanking), and it gives some block (which has some value in multimob pulls).
Progression (physical damage):
Armor > Stam > Block Value > Hit/Agi/Avoidance (depending on encounter, if threat sensitive, hit and agi are better, if not, avoidance better) > Crit > Str > AP > Block Rating
Raid Threat (on a boss that is probably not going to kill you at your gear level, ex: Void Reaver for an SSC/Eye geared tank):
Block Value > Hit > Agi > Armor > Stam > Avoidance
Solo (in Fury Dual Wield Gear, with slow MH weapon)
I have done a lot of math on Prot warrior dps, and it should be based around very high str (for vitality bonus), followed by crit, and then hit. (However hit must be at least at the cap to always hit with specials). The reason Str/AP is best is largely because of victory rush, and gives a lot of benefit to DW. Crit is superior to hit because a lot of our damage is Devastate + Victory rush. And thus once past the hit cap for hitting with specials, crit is better. Thus:
Hit rating below cap to always hit with specials < Str < AP < Crit < Hit < Agi < Armor < Stam