As well you know we got 4 elemental focus feats; fire, acid, electric and cold.

My favourites are:

1- Spirit Of Flame (lots of fire spells, it's no 1)
2-Aqua Mortis (lots of cloud, toxic, acid spells very useful for area damage spells, however it's no 2)

3-Scion of Storms: better than aegis of rime. more useful. but lightning spells are not so much. only chain lightning. it's no 3. it's a "must" for talos clerics. but not my mage's cup of tea

4-Aegis Of Rime (ice storm, ice dagger, cone of cold. I dont use any of them so why spend 1 feat? not useful)