General:
* Clothing
* Attire
* Garments
* Dress
* Habiliment
* Raiment
* Costume
* Outfit
More formal:
* Vestments
* Robes
* Apparel
* Wearing apparel
Informal:
* Threads
* Duds
* Gear
* Rig
Specific types of apparel:
* For men: Suit, tuxedo, uniform, shirt, trousers, jeans
* For women: Dress, skirt, blouse, gown, pants, leggings
* For children: Onesie, romper, overalls, pajamas
The best synonym to use will depend on the specific situation and the desired level of formality.