Melee Crits for 200% damage.

Spells Crit for 150% of base damage.

To see your melee crit percentage open up the skills panel (default "P" key), go to the general tab, and and select Melee from the dropdown menu (patch 2.0 update). Unlike before 2.0 players can now see their spell critical strike % by selecting "Spell" from the menu. It will show info related to casting such as +dmg, +healing and so on.

Each class has a different formula for base critical strike %.