I started the campaign using the DragonQuest (DQ) from SPI. I have a description of those rules elsewhere. They worked well, but I eventually became dissatisfied, probably with the number of independent forumulas that I had to calculate when updating major NPCs and the players had to calculate when updating PCs. These formulas were scattered through the rulebook so it was painful to find them and then painful to actually do the calculation. Steve Jackson Games GURPS 3rd edition rules. (See Section 5.27, “Monsters and Merchants Under Seagate”).
In some ways then it is ironic that the next system used was http://www.sjgames.com/gurps/, which also has lots of things that need to be calculated when characters gain experience. However, at least in GURPS all skills (which includes magic) have costs that are figured off one table, and it is the actual skill number that goes up, rather than the skill rank and then some other number figured by a (often) unique formula.