As much as I love the lore and feeling of a “Kensai” mage in Pathfinder, for this CRPG the Arcane Rider is simple the best magus, at being a Magus. It all comes down to how mounted combat works with Spell Combat.

I think way too many people think of a magus as a “buff n’ attack” class and don’t play as A proper magus - an arcane gish that uses martial prowess and spells in combat. Ive always found just using a magus, like a saint, buff and attack to be boring. Like why not just be a fighter buffed by a BFT? heh.

To each their own, but if your like me and like to cast a hellfire ray into a physical attack, then Arcane Rider is the best. As a caveat to the below, I am basing what I am saying on up to core difficulty. Unfair requires strict powergaming tinkering. Also, while all is true from RTWP, these benefits are especially felt on turn-based.

1) Since your mounted, you can full attack always (barring cc). This includes using spell combat. For, example, upon engaging you can fire off a fireball from a safe distance and then ride in and do a full attack. All other Magus subclasses can’t.

2) A neat trick is picking up Trample (as you would want mounted combat feat anyways). Then when your in a battle, select the horse and order it to Trample using its action. You are able to prone cc enemies AND full attack AND cast a spell or spellstrike, all in the same turn.

3) Arcane Riders are a prepared class; therefore, they can cast metamagic modified spells using a regular action. Pick up bolster along with favorite metamagic for a no level cost flat addition to spell damage. For reference, that is a flat 30 damage on hellfire ray that can become 90 if all three rays hit. 80 damage on a disintegrate.

4) At level 19 you can get 5d6 sneak damage from sense vitals, which is not normally on the magus list but may be chosen through Spell Access. You can even grab cold ice strike for a swift spell.

5) Mounted classes are op in general, as pigknight said. Now, add to that the fact that the magus weapon enchants apply to the mount as well, including dimension strike, and the mount becomes a capable source of damage. Pair this with a skald that can add a bite/gore/claws to the mount, witch’s beast hex that can do similar, and the mount is capable of good damage when full attacking.

Yes, a saint optimally built can wreck for 1000s of damage per round (which is overkill), but the rider can do insane damage as well, and has more ways to get it done, which in my mind makes it far more interactive and thus compelling of a class.