Well you have 2 basic scenarios:
A) The enemy realises that traditionally holy looking "fighters" have a large capacity for healing, dispelling, buffing, and the occasional attack spell and that wimpy robe-wearing commoners are actually bazookas in disguise who can annihilate their entire force if given the chance.
B) The enemy realises that its hungry and these intruders are bothering it. Lunch time.
In A I'd say its a toss up whether to go for the cleric or mage first. First, not every mage is an evoker. It could be one of those trickster illusionists, the charming enchanter, or even just the party buff transmuter. So expending all that effort may not be worth it, without seeing some fireworks first. The heavily armored holy looking guy may just turn out to be a religious fighter, or a paladin, but if you are taking out the cleric youve hit solid gold. Feel free to paralyze the rest of the party, and downed party members may die of bloodloss. Also, youve taken out someone who has a large capacity for inflicting damage every round, whereas a wizard may switch up his magics mid-battle, teleporting away, summoning things (which take a whole round), you can count on a cleric tossing a flamestrike or attacking (while buffed) nearly every round.
Of course, scenario B the monster goes for the closest looking enemy at first, and in later rounds the ones that seem to be piercing its hide more often (the threats). I dont think an owlbear should be able to determine that a wizard cast melf's acid arrow, but it will sure as heck notice the fighter or barbarian in its face hacking away. That pretty much goes on to apply for all levels of play, with monsters (or NPCs) with that much intelligence.
I'm also interested in who ranked above wizard in these polls you were talking about. I mean, obviously cleric. I wouldn't put druid above a wizard or sorceror, and definately not a rogue (how many threads are there about how a high level wizard can out-rogue a rogue? Especially with a couple levels of rogue). With fighters its a toss-up, I see threads with people complaining that wizards aren't viable at high level due to DC nerfs and I see threads complaining that a fighter couldn't possibly keep up with a wizard's damage capacity, not to mention high level adventurers *need* magic - for wizards its a class ability, for fighters it comes out of their gold allotment. The only class I think is better than a wizard is a cleric, and thats dubious depending on campaign because of a lot of monkeying that could happen to your character (if youre into that cool, but its easier to be an independent mage than a cleric).
Wizards have spells like Mage Armor, Shield, Blur, Mirror Image, Displacement, Stoneskin, etc to make up for their low hp with higher AC or spells which deflect/negate attacks. If a wizard wants hp, they have many options (stated earlier) to go about getting them, but I dont think it should be the default for the class.
Technik
