There is no 'best' DeNiro moment.
Every second he's onscreen is superb, whether having dinner with the wife in his little tenement, killing some Don, or touring an olive oil factory in Sicily.
Unlike Pacino who is excellent, riveting, but lets face it somewhat depressing in his role as Michael...DeNiro as young Vito brings a vibrancy to the early segments that is unequaled in either of the films.
There's no 'best' DeNiro moment.
Apple