Ars Magica Rulebook: Ars Magica 5th Edition

Anr Magica 5th edition is witout doubt the best magical system in any fantasy rpg I've encountered in my almost 20 years in the hobby. Detailed yet simple, flexible yet well enough defined for newcomers. It has a much more mythical feel to it than any other, and the setting of a Mythic European Middle Ages gives it a more real feel than some fictional setting.
With the main focus of the game om magic, wizard's and their order, the details of magical research, -study and -invetions is great. It also supplies good mechanics for the adventuring part, along with wide opportunities for non-magical characters.
As a veteran player of 4th edition of this game, and in the know of the game since the 1st edition, this is by far the most optimal iteration.
These core rules are complete by themselves to begin play. altough a myriad of supplements and source books offer new dimensions and expansions of subjects. Which ones are best are up to the different people and playstyles.
Thse rules are easy enough to learn from scratch, or to upgrade from earlier editions. Many of the kinks and inconsistencies have been thoroughly worked out. A lot of things are simpler and mechanics are closely related, to offer an easier overview. A lot of problematic issues from earlier edition are elso better defined now, and make a lot more sense.
Newcomers might have difficulties processing all the details, but the trick is to start out simple, and grow into the system. Veteran players will easily assimilate all parts of it.