Well a Libra could find good traits in anyone if you want an honest answer ;-) Libras are very good at looking at both the pros and cons of every person and sometimes (to their dismay) they simply just can't decide which way to go.
If you want to narrow it down to Sun signs (which you technically shouldn't when you're deciding who exactly to settle down with) I'd say that the most perfect match for a Libra is an Aries. The sign of Aries after all is exactly opposite Libra and opposites certainly do attract!
A Libra needs a good direct Aries to mesh out the uncertainty. Aries are notorious for making quick decisions, however, so a little Libra pondering ends up helping them out too.
Libra is an Air sign and the other Air signs (Aquarius and Gemini) work well with them too since they harmoniously work together. However, you'd really need some sort of "FIRE" (Saggitarius, Leo & Aries) to really create sparks for a peaceful Libran.