Violin Strings: Each violin is different, so I am opened minded to different brands of strings, and even mixing brands. Having said that, my current favorite for myself and students is:
Dominant PRO Strings (violin only at this time) - these were just out in early 2021, and they are a game changer imo: they are rich, powerful, yet subtle and extremely responsive in all dynamics - they speak with a beautiful tone in whisper pianissimo and are beautiful, rich, and clear in full fortissimo AND not screechy on on the E. The best all around strings - I use the complete set myself for my violin. IMO, my wonderful old Italian violin is sounding its best ever! There are options for D and E strings - I currently prefer the Aluminum D over the Silver. For E string, I would avoid the Platinum (too much tension and it squelches the tone on all strings on my violin) The Gold and 2 tin/carbon E string are all excellent.
In the past I have recommend these strings and still do if they sound well on your violin:
Wondertone Solo Strings (violin only), by Pirastro, for A (Aluminum A, NOT Steel A), D, and G strings (not E)
PLEASE double check that you don't buy the Wondertone STEEL A String - it is thinner sounding than the Aluminum A strings and goes bad more quickly!
Evah Pirazzi GOLD Strings, also by Pirastro for E, A, D, and G
E STRING: I use and really like Evah Pirazzi Gold label E strings as well as the Optima Goldbrokat Gold Plated E strings. The Goldbrokat non gold-plated E's are also excellent for most - I recommend the 27 gauge for most students, but I like the 28 gauge for my own violin. They have excellent support (meaning they steel have a sweet tone even with some pressure - compared to other E strings). It is difficult to pin down websites like SHAR for the exact gauge - all they have is heavy or medium from what I have been told, and their reps don't know if those mean 26 or 27 gauge (I think heavy usually ends up being 26). Thankfully, Claire Givens Violins in Minneapolis routinely stocks the Lenzer 28 gauge at a very good price! www.givensviolins.com/
Viola Strings: I find that the best viola strings vary even more for each instrument than violin. Experimenting with mixes of string brands can be very helpful. My current favorite strings are: Warchal Amber for the C,G, and D Strings, and Jargar Forte for my A string, The Warchal have been a game changer for my viola and my viola speaks with a deep voice, while responding quickly and with great color and dynamic flexibility. Warchal strings are not carried by many vendors, but Johnson Strings carries them routinely (Shar does not) https://www.johnsonstring.com/
For Viola A strings: (Try using the same A as the lower strings too) I prefer:
Jargar Forte gauge or regular gauge
Larsen A strings can also work very well and many professional violists use them.
Rosin: There are many fantastic rosins available these days.
My current favorite for both Violin and Viola is Evah Pirazzi Gold Rosin, and my students LOVE the difference in tone it makes on their violins.
I also really like Salchow Rosin for both Violin and Viola. I prefer their darker rosin - it is not translucent like their lighter rosin - many violin shops don't realize there is a difference between their light and dark rosin, but they are both excellent.
The only rosin I really do not care for at all is an expensive green colored rosin (I will refrain from naming it on this site) - if anyone encourages you to purchase a green (and expensive rosin) please check in with me before you waste your money! (pulls very small thin sound out of instruments in my opinion)
Fine Tuners: I am a big fan of using fine tuners on all 4 strings. Using fine tuners on all 4 strings greatly reduces strain on the left hand from tuning, and allows students to more consistently be able to tune their instruments with accuracy. I can’t count the number of times I have students struggling to get their strings in tune and waste precious lesson time battling their pegs, and then not be able to get the strings in tune anyway (I intervene and help out of course, but they need to learn how to tune for themselves). Most professional musicians use fine tuners on only the E string (violin) or A string (Viola). There is a misconception (in my opinion) that the use of fine tuners can negatively impact the quality of tone, but this has not been my experience at all. I started using fine tuners on all 4 strings several years ago, after knowing a friend and colleague at Chicago Symphony who had to get operations on her wrist from the repetitive strain from tuning her instrument pegs (she could play in the Symphony for an extended period for recovery). Ever since I switched to fine tuners, I have had greatly reduced strain on my own hands and wrists, and my violin and viola have never sounded better.