What's the best open auction network for buying unsold inventory?

If you mean programmatic ad exchange / open auction inventory, the “best” network depends a lot on your goals:

Common top options

• Google AdX / Google Authorized Buyers
  Usually the biggest source of scale and demand, especially for display/video. Strong fill and broad buyer competition.
• OpenX
  Known for quality inventory and relatively strong controls; often favored for premium publishers.
• Magnite
  Especially strong in CTV / video and premium programmatic supply.
• PubMatic
  Good premium publisher supply and increasingly strong across formats.
• Xandr / Microsoft Monetize
  Solid marketplace, often useful depending on region and advertiser mix.

If you’re buying “unsold inventory” specifically

For general open-auction buying, the biggest practical factor is usually:

1. Scale/fill
2. Fraud/brand-safety controls
3. Transparency
4. Access to premium supply
5. Price efficiency

Short answer

• For overall reach and volume: Google AdX is often the default leader.
• For premium/curated supply: OpenX, Magnite, and PubMatic can be strong.
• For CTV/video: Magnite is often a top pick.

If you want, I can also give you:

• the best network by ad format (display, native, video, CTV),
• the best DSPs to access open auction inventory, or
• a comparison table of the major exchanges.