It depends on what you prefer. None are exactly the strongest.
Taking Level 2 spells in targeted mode:
- Shock does the most damage of the 'attack' spells at this level, 450 damage over 1.5 seconds.
- Blades does 45 damage per blade with 4 blades, 180 total damage. However, it takes the longest to cast.
- Inferno does 120 damage to the targeted enemy instantly.
Taking Level 5 spells in targeted mode:
- Blades does the most damage at this level, 540 x 8 blades = 4320 damage. However, it takes the longest to cast.
- Shock does 3600 damage and stuns the target for 4 seconds. In surround mode this can chain to 8 enemies.
- Inferno deals 2880 damage to the enemy.
- Vortex has the potential to cause a maximum of 2160 damage over the 6 seconds it lasts for at level 5. However, the damage is only caused if the enemy is hit by, or forced into, an object, and the full amount of damage is unlikely to be caused. It is quite effective in confined spaces and against Hobbes.
- Force Push level 5 does 1800 damage to a targeted enemy, and causes a maximum of 360 extra if they hit into an object. It's good for throwing people, particularly Bandits, off cliffs.
All of these amounts are proportionally reduced in surround mode, and lower levels deal less damage and last for a shorter amount of time.
- Raise Dead can be quite strong, but it depends on the creatures that are summoned. It is only really effective above level 3.
- Time Control does not really come into a strength rating, as it can be useful at all times. Level 2 and above give an edge to all other attacks used. 'Stacking' Time Control at alternating levels allows for the charging of its level 4 and 5 without taking too much damage from enemies - cast level 2 and use its countdown to charge up and cast the level 4.
- Chaos can be useful at level 3 and above when there are lots of enemies around.