Whoa, there are some pretty cool skills in Divinity 2 ^_^ I've not played as a ranger yet, so I'm going to leave that tree out.
So... for the priest, I really love the
Summon Demon skill -probably the most obvious one-. But, you know, when I finally reach the level to spend points on it, I just can't avoid using it like crazy xD I feel somewhat powerful by summoning a dreadful creature. Like summoning Dragons in Neverwinter nights
On the other hand, I'm not very fond of summoning-casters, so this skill is the only one that I spent points on.
For the mage skill tree, I'm unsure. I love pure casters, and I tend to balance the ones I consider best. But I guess my favorite is the mighty
Magic Blast. Even on low levels, it pierces through almost anything. When you reach later stages, combined with Destruction and Battle Mage, it basically kills every poor bastard that dares to look at you
Warriors are not my favorite class in general, but I find Divinity 2 warrior skills pretty rewarding. I doubt beteween Rush Attack and Thousand Strikes... but I prefer
Rush attack. When I played a pure mage in my first playthrough, I hated how enemy warriors were able to reach me with that quick attack, so, when I noticed as a pure warrior that the ability was avalaible for players too, I maximized the skill as a top priority. And I enjoyed it inmensely
When it comes to Dragon Slayer skills, it's pretty difficult to choose. Let's say that dividing battle skills from miscellaneous ones, I would go for
Mind Reading and
Single handed weapon expertise. Mind reading because it is hillarious, and gives a very particular feeling to Divinity 2 universe. Single handed expertise is not the best weapon profficency out there, but I like the speed of it and the looks of my warrior with only one weapon dealing with hordes of enemies
