Since no one makes a model Mark 23 I was looking for the best gas blowback version available.

Although not realistic in weight which brand has the most realistic hammer, slide, hammer drop and safety?

AFAIK there is just one GBB available from KSC.
Tokyo Marui makes a springer and NBB.
For starters read the review below.
It doesn't answer your questions about hammer etc, but it should give you some data to make decision which one is "best" for you.
http://www.justpistols.co.uk/mk23.htm

Another good review:
http://unconventional-airsoft.com/2003/11/14/ksc-mk23-socom-gbb-review/

Actually, the same web pages pop up as first results on Google "mk23 gbb" search:
http://www.google.com/search?ie=UTF-8&oe=UTF-&q=mk23+gbb