The 486 was the last of the "pure x86 core" CPUs. If you want to go for "most pure x86/DOS system," a 486 would be the way to go. I'd even argue that the 486 DX 50 is the "most pure" since it's pre-multiplier - clock speed in is the same as CPU clock speed.
Pentium is where the actual processing core of the CPU starts to diverge from the outward-facing instruction set - the actual processor core, deep at its heart, is no longer "pure x86." Pentium Pro went even further, to the point that it ran 16-bit code (which the vast majority of MS-DOS programs are,) slower than an equivalent-speed Pentium! So no, I would avoid Pentium Pro (686/Socket 😎 for "pure MS-DOS machine." Note that a Pentium II is a Pentium Pro core plus MMX, and also fixed 16-bit performance - so if you were going to go for Pentium Pro, you might as well go for Pentium II.
As for "pure DOS" - even as late as the release of the Pentium II, there were still people running MS-DOS / PC-DOS / DR-DOS as their primary operating system. IBM released "PC DOS 2000" in 1998 as the "non-Windows latest release of DOS." And of course FreeDOS/OpenDOS released versions long after that.
It really depends on what you are aiming for. if you are aiming for "most modern stereotypical processor of the MS-DOS era," I'd go for a 133 MHz Pentium non-MMX. It as the top-end processor that was current at the time of Windows 95's release.