The
best spec is the one you enjoy playing the most.
Magus has a lot of different options in that departement, you can:
1] Play a Single Target spec based around a Bolt of Change (BOC) rotation (Havoc spec) that requires some set-up time and does
heavy
single target damage. Since BOC is undefendable you can basically
delete some players completly in a single rotation (Even some tanks who think they are unsinkable!).
As a Magus your pet can even debuff your own damage type in this spec (Elemental damage)
2] Play a solo roaming, ultra defensive spec using goodies from the Daemonology path to make you a tuff target with high armor, toughness, wounds and damage to kill other roamers in 1vs1 melee distance. You can get so tuff, some players might think they are hitting an actual tank!
3] Play a spec using the Rift skill from the path of Daemonology to rift enemy players to your position, deal aoe damage and help the realm, this spec is also nice to have in warbands, since they can provide you with the heals and guards you need to survive near the frontline. A good rift can break up the enemy position and obliterate their lines.
4] Play a spec based on heavy aoe damage with tzeentchs firestorm and other goodies from the path of changing, defending chokepoints in keep sieges and dealing heavy aoe damage in warband vs. warband battles.
5] Create you own spec that may be a mixture of some others or even something completly new, the possibilities are there.
I only played my Magus to rr50, but
I had the most fun with a Havoc spec, slowly setting up my position, choosing a target, start my rotation and see them go from max health to zero in the blink of an eye, imagining what they are feeling now infront of their screens xD. Could I have done more total damage with aoe? Yes, but Havoc felt really unique for me, I think the burst damage was sometimes even higher than that of a sorc
So based on that, try to find out how
You want to play with your magus and then more experienced players than I am can help you with specific advice.
Magus is a great and fun class to play, with a lot of different options on the table!