If you are looking for instrument flying (lets be honest, the sims will never 'feel' like the real thing) then definately all of the following:
ALL MSFS
PSS DASH 8
FLIGHT-1 ATR72
CAPTAIN SIM 707 & 727
IRON KNUCKLES DC9
As I said, you will never get the true feel, but for learning instrument skills the above are all great.
I feel the PMDG 737 is top stuff, but is more about learning the 737 systems than anything else, same goes for the PSS 'buses' and the PS1 747-400.
Happy Flying
