Choose a transparent qualitative criterion and state it before ranking.

You do not need fabricated precision. Business impact, feasibility, speed, cost, risk, or strategic fit can provide a reasonable basis depending on the case.

You might say you would prioritize ideas with high potential impact and low implementation difficulty, then explain why two ideas rise to the top.

Make the prioritization logic explicit and invite correction if the client has a different constraint.

This helped me stop pretending I had numbers I did not have. Even a simple impact vs feasibility screen is better than random ranking.