Personally, I like the Samsung Yepp. It's about the size of an AA battery. The sound quality seems to be on par with all the best players. They are only 256MB, however, so it won't hold the entire Beatles collection. It's rated to play for 20 hours on a AAA battery and it can hold about tripple that amount of music.
When I was looking at players a couple of months ago, I found a lot of reports of poor variable bit rate (VBR) support and other incompatabilities. That's why I didn't go with the iRiver player. Their stuff looks great so I was kind of leaning that way at first.
I'm really happy with my Yepp, although I found a song that wouldn't play in the very first batch of songs I put on it. It was a 160 kbps fixed bit rate rip that played perfectly on my desktop PC. I was dissapointed at the time but I haven't found any more songs that wouldn't play since then. Most of my MP3s are VBR because that's what my ripper creates by default so VBR support was critical to me. I've found that I can do several workouts before any songs repeat so personally, I don't see the need for any more space, although more is always better.
They all seem to have FM radio so that's not a big issue these days.
When I was reading reviews and recommendations, I found several recommendations of a Taiwanese player that is probably the cheapest MP3 player around. It's a unit that can come with about 30 different brands on it, or it can be totally generic. Best Buy has it as their low end unit. That's a player I didn't try out because the generic nature of it kind of grossed me out but, in retrospect, it might not have been a bad idea because the players tend to get abused when they live their life in a gym bag.