In the sim (without any add-ons), yeah, it is the best. The jets are not for
obvious reasons. The twins, the turboprop, and the complex singles (the
mooney and the 182s) are not good to start on because they have stuff like
props, cowl flaps, and sometimes landing gear to deal with. The antiques are
REALLY hard to fly in many cases, and some are so under powered that precise
attitude control is demanded to avoid disaster. Alot of the older planes
have really unresponsive controls, and will not really teach you any kind of
feel that will help you fly the more contemporary stuff.

In FS2004, the Piper Cub is not a bad trainer once you are in the air, but
take-offs and landings will be much more difficult than the 172, as the Cub
has conventional (read tail-dragger) landing gear. The 172 is relatively
easy to fly, and has all the instrumentation a real world private pilot
would be required to know how to use, so yes, in the sim it is the best
trainer.

In real life, this is a much harder question to answer, but the 172 still
makes a good first trainer choice. Some may say the 152/150 cessnas are
better, which also may be true. I've flown both, and there are good
arguments either way. A 152 is cheaper to rent/operate/buy, and IS a little
easier to stay ahead of, but the 172's controls are alot more representive
of what most light G.A. aircraft feel like, at least those a low-time pilot
is likely to encounter. I like those two as trainers because they are by far
the most likely planes a small local FBO will have for rent.

But, as long as it's not a high-performance (>200 h.p), or complex (flaps,
retractable gear, and constant speed prop),  you can learn to fly in pretty
much anything. Even complex and high performance planes wiil work, although
they will take most people longer due to the higher work load.

In the end, more people learn to fly in 172s these days than any other
plane.