Naturally, your best bet is to start by picking out the strings that match your particular model autoharp. Some of the sets are instrument-specific, like the Rhythm Band ChromAharP Strings, which are designed to fit the RB1545, RB1552 and RB1535. On the other hand, there are general-purpose autoharp strings too: the Oscar Schmidt Autoharp String Set can be used with any 12 or 15-bar autoharp, which makes them a versatile option and a good addition to any autoharp player's instrument. Both of these sets are high-quality strings with great tone and responsiveness, so you can count on your harp to sound its best once you've got them fitted.
As with all things in music, the choice of autoharp strings is a subjective one - it's completely up to you what you'd like to put on your instrument. So don't feel limited in your options! As long as the strings you choose can fit on your autoharp and support the proper tuning, they'll be able to help you make great music for a long time to come. And who knows? With a new set of strings affecting its sound, you just might end up with a harp that sounds better than ever before.