Which troop type should I favor when my research is low?

The right type is the one YOU have researched most — there's no universal "best type". Research on the type you field is a multiplier that applies to your whole army and becomes huge late game.

A concrete example (a typical roster): if Spearmen is your most researched type (#1) and you own 9 dragons green for Spearmen — including centerpieces like a debuff amplifier, an aura Champion for the Center and your single biggest power — then your Spearman backbone is your best axis: research #1 plus green affinity on your best dragons.

The trap to avoid: a well-researched type where almost none of your dragons are green. The right pick is the intersection of your highest research and the type your roster carries best in green.