Zcuzhraj Meats (what a mouthful) has the best smokies and jerky I've ever tasted in my life, the fruit stands have the freshest blood oranges you can find for miles, and The Spice Rack has a mindblowing assortment of hot sauces. Those are my top three, but everyone in the market has something awesome to offer.
Seems like West Side Market is just a curation of small to medium sized vendors, but these businesses have been there for years, and they're all incredibly high quality in terms of service and wares. It's been a few years, and I still have yet to visit every stand. The prices are on the higher side across the board, but everything is locally sourced, and probably the only place you can find certain things.
On most days, this place is my primary reason for coming to W. 25th St.