Which is the most popular left-handed Sigma acoustic guitar?

As with the right handed model, the most popular left-handed Sigma is the 000M-15L.

I'm looking for a good left handed dreadnought electro acoustic with a cutaway. What does Sigma have for me?

Well, Sigma have plenty to suit you so here are two examples, one at a very affordable price and the other at a higher price point: check out the DMC-1STEL and the DRC-28EL. Both are excellent and well worth further exploration!

Are Sigma left handed guitars good quality?

Yes, Sigma make use of excellent materials and woods such as Sitka spruce and mahogany, which all contribute to the gorgeous sound of their guitars as well as their robust construction.