[lch]

Most common is to research the site search spells first (Thau 2, Evoc 2, Conj 3 I believe it was) before doing something specific, but you can gun for something more important to boost your province grabbing, too, like Buff spells (Alt/Ench) for an imprisoned SC pretender, Bladewind/Thunderstrike for Ulm/Caelum and so on. But in general your question can't be answered like this. You aim for something specific, which may be to protect against an enemy or enhance your battle prowess, that's all. What you never should do is to divide research except if next turn the gauge will be full, then you can split off the excess research to another one.