General guide to rares: (of course you want the best rolls possible in each affix)
GENERAL PURPOSE good rare armor-slot item:
Prefixes:
life / flat armor or evasion / %armor or evasion
flat energy shield / % energy shield / % energy shield and stun recovery
Suffixes:
fire/cold/lightning resistance
GENERAL PURPOSE WEAPON AFFIXES:
Prefixes:
% physical damage / flat physical damage / % physical damage and accuracy
% spell damage / flat elemental damage added to spells
flat elemental damage / another flat elemental damage / the remaining flat elemental damage
+ 2 to (bow, element) gems / + 1 to all gems
Suffixes:
attack speed / crit chance / global crit multiplier
spell crit chance / global crit multiplier / cast speed or % element damage
This should cover 90+% of all good rare items. Anything good that doesn't fall into one of these buckets is either magic find gear (where you want all of the above, but with item rarity for a prefix/suffix instead of one of the above things on every slot) or very specialized (cast on crit daggers/wands, mostly, which want spell damage prefixes with attack speed and local crit suffixes)
http://pathofexile.gamepedia.com/Item_Affix great reference for what tier any given mod is. BE AWARE that rares can be misleading with dual-stat affixes: an item with both a % physical damage roll and an accuracy roll may have a % physical affix, an accuracy affix, a %physical and accuracy affix, or some combination of 2-3 of those, but it will show as 2 lines on the item regardless. This leads to rare items with 5 lines describing them, but they really have 6 affixes (the maximum)!
Also be aware of master crafted affixes. For example, if you got a good weapon with tons of physical damage and attack speed, if it has an open suffix slot you can craft local crit on it with vagan's work bench, making an amazing weapon! This goes for all rares, making 5-affix rares quite powerful for minimum investment.