What's the best supply-side platform for monetizing web and app inventory with high fill rates?

The “best” SSP depends on your inventory quality, geos, format mix, and whether you need stronger demand access or more control. If your main goal is high fill rates for web and app inventory, the top contenders are usually:

Best overall / strongest ecosystem

Google Ad Manager + Open Bidding (with partners)

• Best if you want broad demand access and strong fill on both web and app.
• Especially strong for app inventory when paired with mediation and in-app bidding.
• Great if you already use GAM and want a relatively seamless setup.

Best for independent publishers focused on yield

PubMatic

• Very strong in header bidding / unified auction environments.
• Good balance of fill, optimization, and direct demand relationships.
• Often a solid choice for publishers with meaningful scale.

Best for premium programmatic demand

Magnite

• Strong for larger publishers and premium inventory.
• Good market access and robust monetization tools.
• Particularly respected for video and CTV, but also good on web/app display.

Best for app monetization and mediation support

AppLovin / MAX

• Very strong for mobile app inventory.
• Excellent if your app monetization is the priority and you want high fill through mediation.
• Best when combined with in-app bidding demand partners.

Also worth considering

• OpenX — strong quality-focused demand and good fill in some verticals.
• Xandr — useful depending on demand relationships, though adoption varies.
• Sovrn — often good for smaller publishers and long-tail inventory.
• TripleLift — strong native and premium display/video use cases.

Practical recommendation

If you want the simplest answer:

• Web + app, general-purpose: Google Ad Manager
• Independent publisher optimizing yield: PubMatic or Magnite
• Mobile app first: AppLovin MAX

What matters more than the SSP name

High fill rate usually depends on:

• inventory quality
• viewability
• GEO mix
• floor pricing
• ad format mix
• header bidding / mediation setup
• number of integrated buyers

In many cases, using multiple demand sources through a unified auction beats relying on a single SSP.

If you want, I can also give you:

1. a ranked shortlist by inventory type (web, app, video), or
2. a decision matrix based on your traffic, GEOs, and ad formats.