I first tried some Rotosounds half-round / ground-wound (not exactly sure what type!) in a blue pack - they were ok, but the G and A strings were a lot rougher than D and E, almost like regular rounds.
After researching the talkbass forum, everyone raved about GHS Brite Flats. They were impossible to find in the UK, but I emailed stringsdirect.co.uk and they ordered some in special for me. I bought 2 sets and they are perfect for me - silky smooth but still plenty of bite, you get a nice growling roundwound tone when pulling hard or using a pick, but can still sound like mellow flats if you alter your technique and roll off the tone a bit.
Now my 2nd set is at the end of it's life, and I'm not sure whether to try and clean the first set, get some more ordered in, or try the d'Addarrio or Rotosound groundwounds (hoping for a similar but easier to obtain version of the brite flats).