I play a NS Design 6-String Electric Cello and a Josef Lavicka cello. On the electric I use a E guitar string and then D'Addario Helicore strings for the rest with the low F string being a specially made one by them for the electric instrument I'm playing.
On my acoustic I use Larsen Strings. I like the 'soloist' version for the top three and the tungsten wound for the bottom C string.
Are you interested in what strings/instruments because you are playing a cello or guitar yourself?, or just curious?