I just went through all my boot disks, and ran CHKDSK on each one to display the amount of free RAM (out of 640K) on each one, using a totally clean boot (no CONFIG.SYS or AUTOEXEC.BAT):
PC DOS 2.00 ... 630,672 bytes
PC DOS 2.10 ... 630,672
PC DOS 3.10 ... 616,432
MS-DOS 3.10 ... 616,432
PC DOS 3.21 ... 609,392
PC DOS 3.30 ... 600,528
MS-DOS 3.30 ... 600,368
IBM DOS 5.00 ... 593,328
MS-DOS 5.00 ... 593,328
MS-DOS 6.00 ... 592,256
IBM DOS 6.10 ... 593,056
MS-DOS 6.22 ... 592,256
PC DOS 6.30 ... 593,024
PC DOS 7.00 Revision 0 ... 593,840
PC DOS 2000 (7.00 Revision 1) ... 593,760
I also have an MS-DOS 4.01 boot disk, but not a copy of CHKDSK which will work with it, so I can't give it an accurate free RAM amount, but judging by what other utilities report, it's somewhere in the ballpark of 590,000 bytes. So IBM claimed that PC DOS 7 offered the most free RAM of any version of DOS since 3.3, and that does appear to be true!