F-35. The deal was even made.
https://www.defensenews.com/global/...ks-46-billion-contract-for-f-35-fighter-jets/
The subsidy of planes transferred from Bulgaria and Poland are probably 99% covered by the USA.
Ukraine's existing air force infrastructure and pilot training, maintenance organization are based in accordance with Soviet aircraft. Therefore, the most effective and fastest solution for Ukraine was the transfer of Russian-origin aircrafts from the some NATO air forces, which invested in the transition to western standards and continued its investments.
There must still be operational airfields, because Russia still has not achieved 100% air superiority. However, according to a report yesterday, these new aircraft will be stationed in Poland. In other words, they will enter Ukraine through Polish airspace with the Ukrainian flag. Just as Russian troops entered Ukraine from Belarus for the siege of Kyiv, although Belarus did not directly enter the war.
The fact that the airbase will be in Poland may also mean that there will be hundreds of NATO personnel (and full technical support) at that airbase.