I think there's a difference between "best players signed under Txiki" and "Txiki's best signings".
Walker has been fundamental to our success under Pep but we did pretty much just walk into Spurs' office and keep putting money down until they said yes. IIRC we actually could have gotten Walker for 10m cheaper until the Mendy deal happened and Levy wanted the same.
I'd say the same about Rodri. We got so lucky with that transfer, everyone in Spain knew it was too cheap, but Rodri signed for Atleti with that release clause when he was just having his break out season for Villarreal and refused new contracts from Atletico hence us getting him so cheap.
A great deal, but one we got because of Rodri's desire to control his career rather than Txiki being a master negotiator or something.
That's why I'd probably have Fernaninho in there. We went after a guy who had very little buzz around him, hadn't played for Brazil for a decade and was seen a couple of times a year in the CL groups. Paid big money for him but he repaid it 10x.