If you have a housecat, you need a litter box - and you need filler in that box.  Things to consider when choosing the filler include:

• Appeal to cat
• Dust
• Ease of use
• Cost
• Scent
• Impact on environment
  

The ideal litter is, quite simply, the one that your cat will use.  Some cats prefer one type of litter filler over another.  Others will tolerate just about any type of litter.  

There’s an increasingly large variety of litter fillers available:  clay litter, clumping, corncobs, crystals, pine, walnut or newspaper pellets, cracked corn or wheat, the list goes on and on. The two most common types of litter box fillers are clay and clumping litters. 

The regular traditional clay litter is generally the least expensive of the litters. When soiled, it is tossed out in the trash. It absorbs urine well, provides an agreeable texture to the paw. It does a fairly good job of controlling odors; some brands offer a scented version intended to mask odors.  (Keep in mind, though, that your cat has a much more powerful sense of smell than you do, so what smells fairly "nice" to you might be really obnoxious to a cat.)  Regular clay boxes should be changed at least twice a week if being used by one cat. Clay litters can be dusty; you may want to check into the low-dust varieties.

Clumping litters are probably the most common litters used. When liquids are added, the litter forms a scoopable "clump."  With clumping litters, the entire box does not need to be dumped when soiled; the clumps should be scooped out twice daily whenever possible.  This type of litter is required to work with an electric litterbox (and you may need to try several varieties before settling on one that works fairly well). As with clay litter, some brands can be dusty so you may want to check into a low-dust variety.  Several textures of scoopable litter are available: some as fine-grained as sand, some tiny "pebbles" of newspaper, some roughly the  texture of traditional clay litter.  Some authorities are concerned that cats may groom clumping litter off their paws which can clump in their intestines; for this reason, I cannot recommend the very fine sand varieties. (Too, it's been my experience that the fine sand litter sets up like cement and is really awful to clean up if you miss a couple of scooping times.)

Some litter fillers - notably those made of plant materials, wood, paper, etc. - are biodegradable; you can literally drop the clumps into the toilet and flush.  We use SWheat Scoop (a biodegradable scoopable litter with a traditional clay feel) in our boxes upstairs; clay litter is used in the boxes downstairs.

The newest litter is silica pellet litter, and is considered to be the most expensive (the biodegradable scoopable litters are probably the second most expensive). The litter consists of silica "beads" that absorb urine. You stir the litter daily and scoop out the solid waste. When the litter changes color, the litter is dumped out and new litter is used.  Silica pellets generally control odors well.  Note that not all cats like the texture of this litter, as it is very different. It does come in some different shapes, so you may need to experiment and transition patiently.

When changing to a different type of litter, put the litter in a SECOND box next to the first so the cat can choose/adjust.  Or, if possible, mix the two together, gradually increasing the new litter over a period of days until the changeover is complete.

Don't buy just whatever's cheapest or on sale when you're at the store, unless that is the brand your cat prefers.  Your cat isn't going to like standing on a different brand of litter every week.

Be sure to follow the instructions on the packaging concerning how deep the filler should be in the box.

Do NOT use Pine-Sol or Lysol to clean the litter boxes; it can be harmful to cats.  Use a very weak solution of bleach and water (10 to 1) or use simple soap and water.

If you have your cat declawed - which I cannot recommend at all - switching temporarily to a paper litter is a good idea.  It's softer, will expand when wet, and small particles won't stick to the wound.

See related articles:  Choosing the Litter Box and Why Has My Cat Stopped Using the Litterbox??.