All powers tested at maximum "dots" level with a beginner cleric.
Offensive powers:
- Turn Undead: 8 MW, about 10-ish damage to all undead in sight. Causes undead to flee for a few seconds.
- Sticks to Snakes: 9 MW, spits out 8 homing snakes that do about 12 damage on impact, then explode and poison all enemies in a small radius. The poison deals about 8 damage per second, for 3 seconds.
- Glyph of Warding: 10 MW, draws a glyph of fire on the ground that stays for 7.5 seconds. All enemies in a huge radius from the glyph (at least twice the radius of the actual glyph) take about 13 damage 4 times per second. The glyph takes a second to wind up. Can be recast after 4 seconds, so it's possible to stack two glyphs. I couldn't tell if two glyphs actually deal double damage.
- Bull Rush: 20 MW, about 42 damage to a single target. The rush stops if it hits something.
- Flame Strike: 40 MW, a pillar of flame decends on the targeted spot for 4 seconds, dealing about 18 damage twice per second to all enemies in a huge radius.
- Earthquake: 40 MW, sends out two shockwaves with a respectively huge and moderate radius. Both shockwaves deal about 57 damage, the huge shockwave also deals about 6 extra damage that stuns enemies for 3 seconds. Has the annoying habit of not affecting enemies right in front of the cleric.
- Storm of Vengeance: 55 MW, drops 40 chunks of ice spread over the whole screen over the course of several seconds. Each chunk explodes with a small radius, dealing about 23 damage with a high chance of freezing.
Defensive powers:
- Heal: 30 MW, restores health to the whole party. I don't know how much.
- Bane: 25 MW, huge radius. I haven't seen any significant effect on damage dealt. Don't know about damage received.
- Holy Aura: 10 MW, gives a +120 AC bonus for 15 seconds.
- Hold Monster: 30 MW, moderate radius, paralyzes enemies for a good time.
Finishers:
- Heavy Finisher: 9 MW, deals about 66 damage in a small radius. The blow if quite slow to come out, and skitterish enemies (like spiders) tend to move out of its area of effect before it lands. I think this can deal critical damage.
- Fireline Finisher: 12 MW, places a small line of fire in the direction the cleric is pointed. The fire stays for 2 seconds and deals about 17 damage 4 times per second. This damage is probably not melee damage, meaning it won't increase with better Strength, weapons or specialization. It certainly won't deal critical damage.
- Circle Finisher: 15 MW, hits all enemies directly around the cleric for about 47 regular and 3 fire damage. The fire part can also deal critical damage.
If you ask me, Glyph of Warding deals the insane damage to lots of creatures for relatively little MW, and is thereby the most useful power by far. Storm of Vengeance is nice to add on top of that for when you must achieve uberkill as quickly as possible.
I don't know about the defensive ones, but since only 10 MW kills off many enemies in a matter of seconds, I doubt all that much protection is needed with liberal use of glyphs. Hold Monster is quite handy against those annoying fire giants, though not the fire elementals. Heal, Holy Aura, Hold Monster and supposedly Bane also have additional usefulness in multiplayer.