As of my last update in September 2021, there are several popular and reliable online marketplaces, each with its own strengths and focus. The "best" online marketplace can vary depending on your specific needs and preferences. Here are some of the most notable ones:

Amazon: Amazon is one of the largest and most well-known online marketplaces globally. It offers an extensive range of products, fast shipping options, and a convenient shopping experience.

eBay: eBay is a popular marketplace known for its auction-style listings, as well as traditional fixed-price sales. It's a great platform for buying and selling new and used items.

Etsy: If you're interested in handmade, vintage, or unique products, Etsy is an excellent choice. It caters to artists, crafters, and independent sellers.

Alibaba: This Chinese platform is a leading global wholesale marketplace, connecting buyers with suppliers and manufacturers. It's primarily used for bulk purchases and B2B transactions.

Walmart: Walmart's online marketplace has been growing, offering a wide range of products and competitive pricing.

Newegg: If you're looking for electronics, computers, and related products, Newegg is a popular destination with a focus on technology.

Target: Target's online marketplace provides an extensive selection of products in various categories.

Remember that the "best" marketplace for you depends on your specific needs and what you're looking to buy or sell. Each platform may have different fees, shipping options, customer support, and seller policies. It's essential to research each one to determine which aligns best with your preferences and requirements. Also, the landscape of online marketplaces is constantly evolving, so newer platforms may have emerged since my last update.