1. Imperial (pool is rubbish - but are you really going to spend time swimming outside in a hotel pool in Delhi ?)
2. Oberoi (businessy - a bit cold, but very swishy restaurants)
3. Taj

The rest are chains and are pretty much substitutable. Le Meridien is close to the Janpath (i.e. central). Sheraton is out of the way, but in the diplomatic enclave. Avoid the Hyatt.

Really, location should be your guide. If you are touristing, Imperial is ideal. If for business, Oberoi or Taj might be better.