It depends what you're going for:
Unforgivable:
- Killing curse (spell: Avada Kedavra) - kills instantly; painless
- Cruciatus curse (spell: Crucio) - unbearable pain; useful for extracting information from the enemy
- Imperius curse (spell: Imperio) - make the victim do your bidding; useful for infiltrating businesses, can lead to power; allows complete control over situations
According to Mad-Eye Moody, they are "so named because they give you a one-way ticket to Azkaban" - they are technically illegal though used widely in other dark practices.
Attack:
- Stunning (spell: Stupefy) - used to Stun an opponent
- Petrify (spell: Petrificus Totalus) - used to completely freeze opponent
- Leg-locking curse (i forget the spell) - used to lock together the legs of an opponent
- Slicing (spell: Sectumsempra) - used to slice or cut opponent
- Hoist into air (spell: Levicorpus) - used to lift opponent into air upside down; can prove useful
- Disarm (spell: Expelliarmus) - used to get the wand out of opponent's hand
Defence:
- Shield (spell: Protego) - used to protect yourself from attack; can sometimes deflect spells
Other:
- Move objects/make objects fly (spell: Wingardium LeviOSa) - can be useful to make objects attack opponent
- Unlock (spell: Alohamora) - can be useful to unlock doors if in need of hiding place or retreat
