Strings


VIOLIN


For the student, Dominant by Thomastic with a silver D and a Kaplan Golden Spiral E is an affordable and durable set.  For fractional instruments use Super Flexable by Thomastic.   For those looking for more warmth and complexity the Infeld Red set is also an affordable choice. 
For the accomplished player the above sets are often still popular.  Obligato and Evah Pirazzi by Pirastro are both high quality sets.   Obligatos provide warmth and clarity and are often the choice for symphony players.  Pirazzis have great attack and depth with warmth and complexity often the choice for the soloist.  Always use a silver D for violn.  These sets get a bit more expensive!
For fiddlers who want a quick response, Zyex by D’Addario are becoming more popular.
E strings are temperamental.  Fortunately, they are inexpensive enough that individuals can try several out to see which works best.  E strings come in different gauges- the higher the gauge the brighter and harder the sound.   Some E strings worth trying are Kaplan Golden Spiral, Hill, Jargar, Pirastro Silvery, Pirastro Gold, Pirastro Olive, Pirastro Eudoxa and Lenzner goldbrokat.

VIOLA


For the student, again the Dominant set works well.  However, with viola, the aluminum D sting is preferred.
For the accomplished player, again the Obligato and Pirazzi sets are more expensive but worth trying.  For a viola needing more punch in the lower end, a Tungsten Spiracore C can help.  Other A strings to try are Larsen and Jargar.

CELLO


Most cellists demand more bottom end sound with a great singing A string.
For the student, an affordable and durable set that provides quality sound would be Jargar for the A and D strings and Prim for the G and C strings.
Accomplished players often like the Jargar A string for a clear and bright sound or the Larsen A which creates a more voluptuous sound. 
The Larsen Soloist A and D with Larsen Tungsten G and C makes a rich and beautiful sounding set.Strings that Paul recommends –