You can see yourself by going to http://pokemondb.net/move/all and choosing a type from the menu, then clicking to sort the column. But I will summarize anyway...
NORMAL - Explosion
FIRE - V-Create (or Blast Burn for a move more Pokemon can use)
WATER - Water Spout (at full HP) and Hydro Cannon
ELECTRIC - Bolt Strike (or Zap Cannon/Thunder/Volt Tackle for more Pokemon)
GRASS - Frenzy Plant
ICE - Freeze Shock and Ice Burn (or Blizzard for more Pokemon)
FIGHTING - Focus Punch and Meteor Assault
POISON - Gunk Shot and Belch
GROUND - Precipice Blades (or Earthquake for more Pokemon)
FLYING - Sky Attack
PSYCHIC - Prismatic Laser (or Syncronoise/Future Sight for more Pokemon)
BUG - Megahorn
ROCK - Rock Wrecker and Head Smash
GHOST - Shadow Force and Astral Barrage (followed by a bunch more signature moves, then Poltergeist/Shadow Ball is the highest move known by more Pokemon)
DRAGON - Eternabeam (or Draco Meteor for more Pokemon)
DARK - Hyperspace Fury (or Foul Play for more Pokemon)
STEEL - Doom Desire and Steel Beam (or Iron Tail for more Pokemon)
FAIRY - Light of Ruin (unavailable) or Fleur Cannon (or Moonblast for more Pokemon)
SHADOW - Shadow End
Remember these are the most powerful only, not the actual best moves. For example Frenzy Plant stops you from moving the next turn so it's not a good move at all. Also it doesn't include Z-Moves, many of which end up more powerful. This list also ignores OHKO moves, and focuses on moves with base power.