Aren't most BASE jumps done without a reserve?

Yes. This may be changing - I no longer actively BASE jump so I have
not kept up.

Do BASE jumpers prefer
the reserve square or the sport chute (since control into tight LZs
is
a consideration)?

BASE jumpers prefer a parachute designed for BASE. Before these were
available, most BASE jumpers used parachutes normally used as reserves
(the Raven series were quite popular). Some still use them. None used
anything even vaguely similar to the modern sport main canopy.

Michael