As I mentioned above, every Sign has its use. Still, after experiencing mutation, I found Aard to be the coolest and the most useful. Igni slowly goes on the back burner as you level up, and Axii is used for fun encounters. Quen and Aard end up proving to be the best ones, Aard against a group of foes and Quen against strong enemies for long battles. Yrden is especially useful against Wraiths but to get the perfect build, and you'll probably be at the end of the game. To follow the upgrade path, be sure to upgrade Aard. Suppose you're ever unsure about your upgrades. In that case, many merchants sell a reset for ability points so you can buy them and re-distribute them for a new build.