Ipod Classic: For it's feature set and price, I believe the current Classic is superior to any alternative.
Ipod Touch: I believe it's better over-all than the Q5 but many disagree and the Q5 has more options.
Ipod Nano: The Nano is a weak link. I strongly feel the Sony's are far superior flash-based players. If we are just looking at sound quality, then there is no argument--the Sony's win hands down and by quite a bit.
Ipod Shuffle: Another weak offering. Again by sound quality, the Sansa Clip is the obvious winner here and by quite a bit--many feel the Clip is the best sounding un-amped mp3 playe on the market.
Other notable mentions: The Cowon D2 is remarkable (amazing sound and 50+ hours of battery life!); the Iaudio 7 is notable for its sound quality;
Biggest Ipod weaknesses:
-Crappy format support resulting in mediocre sound (no FLAC, etc).
-Average battery life
-Pricing on some (Shuffle and Nano are over-priced).
-Many PMPs have SD-slots and some prefer this, as it allows for cheap 16gb+ players with infinite rotating selection.
-No FM tuner for those who care.