...since I can't find any of the last 56 or so threads dedicated to the topic.
To start, I'm going to toss up Algid Enhancement, from Frostburn. This is a 6th level cleric spell that grants a deflection bonus to AC, an enhancement bonus to attack rolls, 1d8 temporary hit points, and a +2 resistance bonus on saves against fire. Here's the good part: those bonuses scale with level, increasing by 1 for every 3 caster levels. Here's the better part: there's no cap. A 20th level caster is going to get a +7 deflection bonus to AC, 1d8+6 temporary HP, a +7 enhancement bonus to attack rolls, and a +8 bonus on saves against fire. Combined with CL-boosting tricks (not that anyone on these boards ever uses CL-boosting tricks!) the bonuses can get obscene.
Here's the gravy: it lasts 24 hours, and it affects one creature PER LEVEL. That's one HECK of a deal for a single 6th level buff spell.
Now, there is one catch: it only works on creatures with the cold subtype. That's not such a big catch, though, given that there are many ways to get the cold subtype. Mantle of the Icy Soul is also a 6th level cleric spell that does just that for an hour per level. Extend it, and it'll last as long as Algid Enhancement right out of the box. (It might do that anyway, if you're significantly boosting your CL.)
About the only thing that would make this better would be a cost-efficient way of granting a large number of creatures the Cold subtype all at once. Any suggestions?