help me choose the best MP3 player!

From what you've described above, two players would be the best bet for you: a hard drive player and a solid state player.

For situations where you will be give a hard shaking to the player (like when you are working out at a gym), the best choice would be a solid state player with a minimum of 128mb. This would give you about 2 hours of music (at 128bits) and should allow you to carry enough music for a workout with songs to spare.

With no moving parts, shaking would not damage the player and you would not have to worry about skipping. As an added benefit, it would have good battery life.

For other uses (such as when using it in a car, or while walking around), a hard drive player would be the best choice. They have the capacity to carry a massive amount of music. Battery life tends to be an issue, but extra batteries, and/or an external battery pack can deal with this.

Based on what you mentioned, you will need an FM transmitter to listen to your music through your car stereo. You may also need a cigarette light adapter to power/charge the player while you are driving. Some manufacturers have added the ability to connect your iPod through their car stereo.

I hope that helps.

Yes, hard drive players can survive the gym, but you will have to excuse skips here and there. I'd suggest a microdrive, they have the memory you need and fewest skips. As for which player, I'd look into the Zen Micro.