Yes i know asking for the best one is a difficult question because you don't know what i like and such, which is why i also asked for the most popular.

http://www.tesnexus.com/downloads/file.php?id=12466 which keeps a more vanilla feel to the game's magic while adding a significant number of new spells and tweaking several of the default ones. Also re-balances the magic system to make it less exploitable.

http://theelderscrolls.info/?go=dlfile&fileid=219 a very aggressive re-balancing of the magic system with an emphasis on long-lasting passive spells. Also includes plenty of new spells and a number of interesting goodies for anyone who uses the sister mod, Race Balancing Project.

You can even use the two together. Simply follow the instructions for doing so, and load whichever mod you want the spell balancing to take priority from after the other.