Best solo strings sounds?

Best is of course subjective, and I actually like the default string sounds in MuseScore better than almost all other alternatives I've heard. The optional "HQ" soundfont you can get as an extension in Help / Resource Manager has some further improvements though.

But if you're wanting to try other soundfonts, just do a web search to find some, then install them as per the Soundfonts section of the Handbook. Note however many third party soundfonts won't support single note dynamics, so even if they are better to your ears in some respects, that may turn out to be a deal breaker overall.