Hi Brian,
In no particular order actually:
Level-D 767
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rocksolid simulated aircraft. Handles beautifully and deserves all credit.
Maddog 2006
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not mentioned in any of the above posts, but a serious challenge where you really need those checklists! Real icing (ALSO with FS failures switched off!), and follow correct takeoff procedures or you'll end up with an enginefire on the runway! Great and very complex system simulation. Imagine: it's even possible to pull fuses! Make sure to get the 2006 version.
Ultimate Traffic
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bit older already perhaps, but populates the sky and airports at least with realistic traffic from real world schedules. Not like the other one, where you could end up with A380's on your local airfield...
PMDG 737 series
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need i say more!?!?
FSbuild
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Not always very intuitive to handle (although after a while you'll get the hang of it), but the very best and most advanced flightplanning tool available.
Further would further like to agree with Jonathan on ActiveSky, but you allowed me only to name five...

All above is (perhaps unfortunately?) payware, yet my personal opinion is payware usually (NOT ALWAYS THOUGH! Am also one of those who has been fooled by the SSW A310...) pays itself of.
Keep the blue side up!
Stef
EHM-1242