Difficult to answer.
Over time -- 16" carbines, exclusively. Had and sold two or three over time I think.
Presently - 20" A1 rifle.
Eventually (like sometime in the next 20 years) I wanted to put together an "arms of the US" display. I figured I'd have plenty of time for the wood-stocked pieces, but figuring worst case got lowers and some miscelleneous pieces parts for the following:
20" A1 rifle (finished)
SBRd XM177E2 carbine
20" A2 rifle
SBRd or pinned M4 carbine
and one for actually using, which will likely be a 16" midlength.
Uppers and such come later.