What's the best ad exchange for auction-based inventory trading across premium web inventory?

If you mean auction-based programmatic trading for premium web inventory, the “best” exchange depends on whether you’re buying or selling and what you optimize for:

Common top choices

• Google AdX / Google Ad Manager exchange
  Often the strongest for premium web scale, liquidity, and demand breadth. Very common default choice for publishers with quality inventory.

• Xandr (Microsoft Monetize / formerly AppNexus)
  Strong for open auction efficiency, controls, and transparency, especially if you want more direct exchange-style programmatic trading.

• Magnite
  Particularly strong for publisher-side monetization and premium supply; widely used across web inventory.

• Index Exchange
  Known for premium web focus, efficiency, and clean auction mechanics. Often cited as a top exchange for high-quality web inventory.

• OpenX
  Good for premium web and curated supply, with solid demand relationships.

If you want a single practical answer

For premium web inventory in auction-based trading, Index Exchange and Google AdX are often the first places people evaluate, with Magnite and Xandr close behind depending on setup.

Rule of thumb

• Publisher maximizing revenue: Google AdX, Magnite, Index Exchange
• Buyer seeking premium inventory access: Google AdX, Index Exchange, Xandr
• Need transparency/control: Xandr, Index Exchange
• Need maximum scale: Google AdX

If you want, I can also rank them by:

1. publisher monetization,
2. buyer access to premium web supply, or
3. fee/transparency/ease of integration.