I play baritone ukes pretty much exclusively.
Some of my favourite strings for a Baritone include Worth fluorocarbon.
Worth also have a non-wound set, which can be used to tune to re-entrant DGBE.
I like Worth because they have moderate tension, and a crisp, projecting tone.
I also really like Koo'lau Alohi baritone strings.
The strings are a bit thicker, which gives it a fat, mellow tone at a moderate tension.
Great for fingerpicking, but it also has quite a nice boom-boom for strumming.
Martin Fluorocarbons - someone has mentioned it.
I like the tone and sustain, but I always felt that the strings were perhaps.. a bit too light.
They feel very thin and low-tension, which some people really like. Personally, it felt a bit too light-on for my liking.
D'addario nylon baritone strings give a sound very similar to a classical/flamenco guitar.
I suppose being a huge guitar-string manufacturer, it's no surprise. It has a bit less volume and sustain compared to fluorocarbons per my experience.
Not personally a huge fan of Aquila baritones. I like Aquila nylguts on soprano, concert and tenor.. but have been somewhat unhappy with baritones. The nylguts don't really project well for me and feel a bit thin/floppy.
I've also used GHS and Aquila/D'addario Nyltech baritone strings.
They were "ok" strings, but not ones I'd likely purchase again.
A special string I've used is the Guadalupe "low octave" GCEA strings.
Basically it tunes your baritone to GCEA one octave below normal GCEA (with a low G).
It's an interesting tuning.
The same company also has a DGBE regular tuning set - that's on my list of strings to try next.
The Guadalupe strings are all wound, and have a springy, bouncy feel. Excellent for fingerpicking, so-so for strumming.
It makes your uke sound a bit like a "harp" - very strong fundamental and great sustain.
Here is a video I use the Guadalupe GCEA. They are available for puchase at Mainland ukulele website:
http://www.youtube.com/watch?v=HCyLRHl0wEY