What MAKE of strings is best?
General mountain dulcimer or music discussions
The general string gauges will allow you, generally speaking, to tune from the key of C tunings (CGG, CGc) up to maybe key of F tunings (FCC, Fcf). Below C they will be too floppy and buzz and sound mushy, especially on the thinner strings. Around F, and certainly G, you risk breaking the bass string although the thinner strings will easily tune.
No one set of strings will allow you to tune from A to G. If you're tuning below the key of C you need new strings (most of us get a second dulcimer along with the strings). Likewise if you're tuning to the keys of F and G primarily.
Not wanting to rig a dulcimer with a separate set of strings to play the key of G - GDD, GDg etc. is what has given rise to the popularity of the so-called Reverse Ionian tuning DGd. That tuning has the same notes, but not the same 'flavor' if you will, of real Ionian key of G tuning -- GDD.
I have one dulcimer that has a 27" VSL which I keep strung with 18-11-11 and tune to GDD. Another is strung with 22w-12-12-12 and tunes back and forth to the keys of C and D tunings