Anson Winder, Skyler Halford, Craig Cusick, Brock Zylstra, Logan Magnusson, etc. None of these guys were stars, and were particularly non-contributors as under-classmen, but they provided some badly needed depth and each of them had key moments when they led us to a win or two as juniors and seniors.
It's a bit different for bigs, but Noah Hartsock and Keena Young showed almost zero promise as under-classmen... and they won the CPOY by their senior years.