Honestly what sells the best is what you are passionate about. I was successful selling hand painted items, jam and now crochet items.
One field I would not go in unless I had something really unique is jewelry. This is a flooded market. Normally I don't pay attention to that as many feel crochet is flooded too and I do quite well. But hand made jewelry truly is bursting at the seams. But again if you have something super unique and you are passionate you may still do well.
A little nice field that I think will always sell well is accessories. Scarves, gloves, hats etc. They can be sewn, knitted, crocheted etc. As long as you stay up on trends and colors you can always do well with accessories.
Baby items will always do well because people are always going to have babies. Personalized baby items sell even better. I know in crochet I get asked a lot for handmade baby sweaters. I don't make those but they do sell very very well. Baby booties are another big seller. I know when I have baby hats in my Etsy shop they sell like crazy. And of course I make and sell a ton of baby blankets. Other big sellers are car seat covers, baby room decor, and hand made baby clothes.
Another craft market I see exploding right now is hand made birthday party favors. This is for people that want that handmade decoration look without all the work. People spend huge amounts on their kids birthday parties. And birthday parties happen all year long. If you like cutting and gluing paper this is for you.