I think both the 20” and the 6720 will have some collectibility due to both having “ar15” in the roll mark, same with the 6721.
Mentioned above, the socom will. The 2017 should be more desirable over the 2013 version of the “m4a1” roll marks. The stamped US property will be more desirable over the engraved ones.
The 6920 lowers that Brownells had will have some collectibility due to being pistol eligible at least until either SBR laws change or new laws put some kind of end to ar pistols.
While not a complete rifle, the “COMMANDO” lowers AU sold will be collectible and in my opinion maybe the most collectible of modern Colts. There is only around 150 of them out there, they were pistol eligible, and one of the coolest roll marks.