For each bond in the basket you can compute the financing rate at which the cash-and-carry-arbitrage breaks even. The bond with the highest implied repo rate is the cheapest-to-deliver, because it earns the most relative to its cost of carry.
Comparing the implied repo rate with the actual repo rate in the market tells you whether futures are rich or cheap. If implied repo sits well above actual repo, buying the bond and selling futures is profitable, which is the entry signal for a treasury-basis-trade.
Example: buy a note for $101,200 all-in, receive an invoice of $101,850 in 60 days plus $420 of coupon accrual. Return = (102,270 - 101,200) / 101,200 x 365/60 = 6.4% annualised. With actual repo at 5.3%, the trade earns 110 basis points of carry.
Related: cheapest-to-deliver, treasury-basis-trade, conversion-factor, cash-and-carry-arbitrage, cost-of-carry