Try DR strings. They have some really good ones called "Lo Riders". Marcus Miller also has a signature brand of strings with DR called "Fat Beams". If you use your strings for very long periods of time, get the DR "Black Beauties". These are coated strings designed to last about three times longer than normal strings. However they are more expensive. DR strings are perfect if you like to a have a deep growling tone to your bass or you do a lot of slap. If you like Marcus Miller's tone you'll love DR.
If you want to have a more warm, organic tone, try some nylon tapewound strings. GHS makes some very good ones. This kind of string is good if you want your tone to be more like Pino Palladino or Raphael Saadiq. Another advantage of nylon tapewound strings is that they don't corrode. I think they can hold their tone forever.