DMs 6-13 are negligibly different. 9-13 even more so (that is to say even less performance difference between them). Pressure differences have been too small to matter. DM9 is first of the shorter length martixes. Since then whats changed has been minor if not non existent in terms of performance. The dm11 got the NT style grip frame and new ASA, 12 and 13 have the windows and tool free bolt removal, but they all operate the same as the dm9 did.
So I wouldn't say the 9 is the best overall. But it is certainly the best value for money when it comes to buying one used, as it is at its core the same as every DM that has come after it.
-value for money
-comfort (personally speaking)
-no eye covers
-not the most efficient (can be improved with mods, but still not as good as most other markers, also counters the smoothness aspect)
-should be lubed more frequently
Personally I prefer the DM11 mostly because I prefer the asa and gripframe/tool free grip removal, but it doesn't shoot any better or worse than a dm9.