The choices you listed are vary different. It all depends on what you want or need.
They all do video. Gigabeat and Zune have larger screens for that.
Do you want to use it as an external hard drive? Ipod and Zen Vision M can do that, but the Zune and Gigabeat can't.
Want FM radio? That rules out the iPod.
Are you planning on purchasing music and videos? If you opt for the iPod or Zune you are stuck with iTunes or Zune Marketplace for purchased content. You also have to use their software to transfer content to your player. The Zen and Gigabeat support Plays For Sure, so you can purchase content or do a subscription from several different sources.
By the way, the Zune Marketplace isn't selling videos yet, but will probably do sometime in the near future.
So once you know what features you want, you may be able to narrow it down. Hopefully, you can go into a store so that you can actually see and hear them.
I have several iPods, including the 2nd Gen Nano and 5.5 Gen Video. I like the user interface, sounds great (I'm not an audiophile - others may tell you that it sounds crappy ), and iTunes is easy to work with. I purchased a $49 accessory that adds FM radio, but rarely use it.
Which MP3 player should I buy?
Zen Vision M 60GB
Toshiba Gigabeat S 60GB
Microsoft Zune 30GB
Any advice would be great!