I will repeat a lot of what was already posted but her is my opinions, I have or have had all these and my findings are:
Ipod, great interface, short on features, not that great sound quality
IRiver, poor interface, many features, excellent sound quality
Creative, good interface, many features, excellent sound quality, 2nd only to IRiver.
Also as for smaller players which you didn't seem to ask about, but I have an Ipod Shuffle as well, and it is basic and with not good sound quality at all, but that is what it is good for, for me maybe dropping a couple podcasts on there and mowing the grass and it is well suited, it is very durable. Now I just bought one of the little Mobiblu cubes DAH-1500i (I think) and this is a superb sounding little player with great features but only 1GB so it probably won't fit your needs.