Might as well add my priorities for nature research:
Alt. 6 - Mass Protection
costs a gem, but everyone gets barkskin - great for turning medium armour units into high-armour units
Conj. 4 - Strength of Gaia
Conj. 5 - Howl
Strength of Gaia gives side benefits as well as increasing your nature power, and howl is a nice way to harass casters since you can't target them anymore
Ench. 4 - Haste
Ench. 6 - Relief
Haste is nice if you're fighting an enemy big on missile weapons
Thaum. 5 - Growing Fury
Thaum. 7 - Charm
Growing Fury makes your guys go berserk, which means they won't rout. This is good
Charm is high-level, and short range, but otherwise good news
Construction (in general)
Everybody likes construction - specifically for treelord's staff or thistle mace to boost power, and if you have air various anti-missile items. Re-invigoration items are always nice too.
I don't much go for evocation, unless I'm making a poison-resistant army - which takes a lot of gems for summoning, and isn't always compatible with your regular army.
just my 2 cents