Well, how about the 4859? This locomotive is noteworthy in three regards:
1) It powered the first GG1 hauled passenger train from Philadelphia to
Harrisburg in 1938.
2) As the Conrail 4859, it was assigned to the last freight train powered
by G's in 1979.
3) The locomotive, restored to its DGLE pinstripe livery, is now on display
at Harrisburg station although I consider this item much less significant
than the previous two points.
Even though it doesn't garner its fair share of attention, another very significant GG1 is the 4879 as it was the last G assigned to a regularly scheduled revenue passenger train. The date was October 29, 1983 and the train was NJ Transit North Jersey Coast Line Train 3323. I was commuting into the city at the time and yours truly had the honor of being one of the 4879's passengers that evening. The 4879, along with the 4877 and 4882, participated in NJT's GG1 commemorative festivities and "last run" the following day. The locomotive is currently in the collection of the United Railroad Historical Society of New Jersey.
There are other notables but these are a few which stand out for me despite them not having received much recognition within three rail O gauge circles - all the more reason to pick one of these G's to impress your friends.
Bob