What is the most powerful beam spell combo?

It's quite a straightforward and well known weakness of Magicka's design - the steam/lightning/arcane beam.

I'm also reasonably sure that since lightning gains the great bonus from wet targets, having it on three instances instead of two would give you a slightly more effective beam. Id est, Steam+Lightning+Lightning+Lightning+Arcane or "QFAAAS".

Tomasz's answer is not quite correct. What Lightning does against wet targets is multiply the total spell damage by 2, not just the lightning part of it. This is why lightning+arcane combos are extremely effective - the arcane damage is doubled as well against wet enemies.

There are a couple contenders for "most powerful" beam. The highest damage per second is quite similar to the one that you're currently using - [QFQFQFSA]. Its nickname is the Super Steam Lightning Beam.

the second does slightly less damage - about 2% less - but lasts 4 seconds longer, which can be useful at times. This can be conjured by replacing a lightning with arcane in the above combination [QFQFASS]

The two above spells start on the same principle - The steam will instantly wet the target while dealing damage, and the lightning will cause the entire beam damage to be doubled. Adding steam elements increases damage slightly faster than adding lightning elements, while adding arcane elements does not increase damage at all, only duration.

One final contender for "most powerful" comes from its incredible knockback power, not the damage per second. If you are fighting an enemy Wizard, it is often safer to deal less damage per second but keep him from casting spells back at you. [SQQQQ] will cause high knockback to anything you hit, preventing it from easily casting spells. If you can trap an enemy against a wall with it, you can eliminate them with total security. This beam has been called the Fire Hose or Riot Cannon.

Source - Magickapedia

http://www.magickapedia.net/index.php?title=Spell_Combinations#Beams