Musicians have the most influence in the sound of the music they produce. But to make it easier I have recommendations for strings.

For small size instruments:

Superflexible by Thomastik Infeld
Sometimes on ¾ quarter size violins, full size strings may be used, in which case we recommend Dominant strings (using a silver D) with a Kaplan E.

For full size violins:
Fiddlers often like to use steel strings, in which case I recommend Thomastik Infeld’s Superflexible.
For classical musicians we recommend Thomastik Infeld’s Dominant strings (using a silver D) with a Kaplan E.
For serious classical players, Evah Pirazzi is an expensive but worthwhile venture.

E strings are temperamental. Fortunately, they are reasonably inexpensive, and so I recommend trying several brands to see which one works for you. E strings come in different gauges, and the higher the gauge, the brighter and harder the sound. Here are some brands that I recommend trying:

Kaplan
Hill
Jargar
Pirastro Gold
Pirastro Eudoxa
Lenzner goldbrokat

Viola:
I recommend Dominant strings using an aluminum D. You may also use the Dominants with a Larsen or Jargar A. For violas that need a little more punch in the lower register, a tungsten C may be used.

Celli:
Most cellists demand more bottom end sound with a great singing A string.

For the top A and D strings:
I recommend using Jargar for a clearer and brighter sound. For a “fatter” and more voluptuous sound I recommend using Larsen soloist for the A and D string.
For the bottom G and C strings:
At this time Larsen has the best G Tungsten.
Larsen Tungsten C has a fatter sound that is not as bright as the Spirocore Tungsten C. Spirocore Tungsten C is a bit clearer and more powerful. Spirocore Silver C & G may also be used, but they don’t have quite the punch of Tungsten strings.

A good string setup for a student is: A & D Jargar on the top strings, C & G Prim on the bottom strings. Helicore Tungtsen C & G also work well.