Probably my
ultimum desideratum in a "51" is a DJ Vacumatic filler in Blue Cedar with a lined sterling cap with perfect plating. Absent that pen, I'm deeply enamored of my DJ Blue Cedar with a Lustraloy cap (which pen I refer to as
the "51" because it's the pen that I always have with me):

I also like Cordovan Brown with a GF cap, like this one:

Moving into aerometrics, I gotta say I love the Flighter:

(I'd like it even better if it weren't so doggone easy to put wear marks into the barrel with the cap!) After that, I really don't think you can beat Plum with a GF cap:

I don't actually
hate any of the colors, although I really don't have any great fondness for Mustard (which I believe Parker called Yellowstone). For a long time, I thought I couldn't live without a Nassau Green "51", but that lastged only until the moment I laid eyes on a MIB DJ Blue Cedar with lined sterling cap...