cleric lv 3, magus lv 7, mystic theurge lv 10. we get a 17th level caster in medium armor with full access to the theurge list, and lv 7 divine magic. It gives us a very offensive theurge, bab +12/7/2, and a decent armor class (especially with a light mithril shield). mithril full plate would be ideal. cleric+magus spells together provide a unique presence on the battlefield, with divine magic covering up the problems created by the magus' small spell list.
rogue 3, magus 7, arcane trickster 10. here we end up with a sneak-attacking magus in medium armor, again casting his 6th level spells as caster 17. bab is 12/7/2, and 5d6 sneak attack. tricky spells and invisible thief let the caster get into position with a sneak spellstrike. This is a more combat-oriented trickster, and much more survivable than a wizard or bard multiclass trickster.
on the other hand, magus 4, rogue 6, arcane trickster 10 is the other end of this scale with an extra 1d6 of sneak and another rogue talent.
eldritch knight someone else has already looked at in detail.
Loremaster would be interesting, but there are only 5 divination spells on the list [arcane sight, detect/read magic, true strike, true seeing]. the lack of 'see invisibility' may be a problem, and i suspect it should be added to the spell list as it can be very useful to combat. multiclassing wizard 1 would fix this, but defeat the purpose of being a magus in the first place. multiclassing a level of bard could also solve this, which does stack nicely in light armor but would preclude the use of medium armor.
Magus 8/bard 2/loremaster 10 in light armor with bab +12/7/2. versatile performance can be quite useful, and the bard levels can add cure light wounds to his spell list, a nice addition to an effective 18th level caster magus. add in the magical knack trait, and we keep max out the magus caster level at 20. a mithril breastplate would be the best armor in this case. medium armor would preclude the use of bardic magic, while keeping the availability of wands of CLW, which may be worth it.
magus 10/arcane archer 10 would get 17 levels of casting, 17/12/7/2 bab, more skills and hp, and be able to weapon specialize in his bow. the requirement is 3 feats that every archer magus should have anyway, and a magus 16/arcane archer 4 build could work quite well, losing only 1 level of spellcasting and gaining a 4th attack from the higher bab.
Magus is also very useful if you grab 1 to 4 levels for a class that is in light armor anyway, such as a rogue. a rogue 16/magus 4 would gain access to magic in light armor(with a high dex so medium wont work that well anyway), no loss in BaB or hp, effectively trading skill points for arcane magic which can stack very nicely with sneak attacks in melee. the downside is the loss of a couple rogue talents, 2d6 of sneak, and master strike.
i dont have a copy of the APG yet, but those six combinations are what would work from the core rules. anything else would be losing too much to gain too little. the magus spell progression fits in surprisingly well with most of these builds, keeping the ability to cast lv 6 spells in medium armor in most likely cases.
