Flight Simulator 2002 Professional Edition is what I use now.
In the past, I've used Flight Simulator 98, Flight Simulator 2000 Professional Edition, and Sierra Pro Pilot which I recently popped in to bring back some memories.
I gotta say, the start-up sounds in Pro Pilot are amazing. Much better than in the Flight Simulator series so far.
I hear FS2004 will have amazing start-up sounds which is good.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran