I’d say the following are must-haves, but if you scan the links below you’ll find some helpful descriptions of each and what they are good for (moonlight, water effects, sunrise, etc.):
(And for you computer geeks out there, NO, YOU CAN’T SHOOT WITHOUT GELS AND GET THESE EFFECTS IN POST!   )

Each shooter will have a unique selection, since you might be doing a lot of nighttime scenes, or interviews, or shooting under fluorescents, or who knows?

General
Cinelux #18: Flame
Warm pinkish amber. Afternoon sunset. Good sidelight. (Transmission = 56%).

Cinelux #318: Mayan Sun
A medium salmon color which evokes feeling of a tropical island. A good sunset color. Interesting backlight and accent color. Good for warm tonal effects. (Transmission = 52%).

Roscolux #02: Bastard Amber
Good where a tint of color is needed. Excellent for natural skin tones. (Transmission = 78%).

Roscolux #67: Light Sky Blue
Excellent sky color. Useful for cyc and border lights. (Transmission = 26%).

Roscolux #70: Nile Blue
Useful for very light midday skies. Occasionally used for general cool tint. (Transmission = 45%).

CTB gels:
E-Colour #201: Full C.T. Blue
Tungsten to Daylight 3200K to 5700K (Transmission = 34%).

E-Colour #281: 3/4 CTB
Tungsten to Daylight 3200K to 5000K (Transmission = 45.5%).

E-Colour #202: Half C.T. Blue
Tungsten to Daylight 3200K to 4300K (Transmission = 55%).

E-Colour #203: Quarter C.T. Blue
Tungsten to Daylight 3200K to 3600K (Transmission = 69.2%).

ND gels:
Roscolux #97: Light Grey
Neutral greys to reduce intensity without affecting color temperature. (Transmission = 50%).

Roscolux #397: Pale Grey
A half stop neutral density. (Transmission = 70%).

Diffusion gels:
Roscolux #100: Frost
Frost. Changes ellipsoidal to fresnel beam pattern.

Roscolux #104: Tough Silk
Creates a slash of light for stretching light along stairs, tables, cycs, etc. Diffuses while maintaining compactness of beam.

Roscolux #116: Tough White Diffusion
A range of diffusion materials adapted from cinematography-used for softening the shadow of the beam while maintaining a relatively high color temperature because of the use of ultra-white pigments in the manufacturing process.

I didn’t include any saturated color gels if you are shooting club, music video, or want unmotivated intense color swatches for interviews.

http://www.rosco.com/us/filters/roscolux.asp

http://www.rosco.com/us/filters/cinelux.asp

http://www.rosco.com/us/filters/supergel.asp

http://www.rosco.com/us/filters/ecolour.asp

http://www.rosco.com/us/filters/calcolour.asp