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 - Hydro Cannon
ELECTRIC - Bolt Strike (or Zap Cannon/Thunder/Volt Tackle for more Pokemon)
GRASS - Frenzy Plant
ICE - Freeze Shock/Ice Burn (or Blizzard for more Pokemon)
FIGHTING - Focus Punch
POISON - Gunk Shot/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/Head Smash
GHOST - Shadow Force (followed by a bunch more signature moves, then Shadow Ball is the highest move known by more Pokemon)
DRAGON - Roar of Time (or Draco Meteor for more Pokemon)
DARK - Hyperspace Fury (or Foul Play for more Pokemon)
STEEL - Doom Desire (or Iron Tail for more Pokemon)
FAIRY - Light of Ruin (not actually available) or Fleur Cannon (or Moonblast for more Pokemon)
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.