Choice of output file format will be a key determining factor. If you're planning on using a lossy format like MP3, WMA or AAC you're already behind the eight ball, particularly when it comes to sound quality over a set of external speakers. Whereas through headphones the lossy files are harder to discern, any decent set of speakers--especially with reasonably good amplification--will bring out the sonic limitations of compressed, lossy files.
The data compression will be apparent in the lack of spatial depth and imaging in the resultant sound, even at the highest bitrate rips for those types of file formats. It takes quite a bit of signal processing (re: electronics trickery..and usually lots of cash to go with it) in order reprocess lossy files to make them sound acceptably good through speakers.
Going lossless (WAV, WMA Lossless, Apple Lossless, etc.) will result in better-sounding output, but even there you're playing with fire because it still depends A LOT on the quality of soundcard and encoding software that you use. Lossless also results in much LARGER data files, so if you go this route storage capacity will INDEED be a concern, depending on the number of files you'll need to carry.
If lossless is used, I would definitely recommend a hard-drive based device, or a large capacity flash-based player (32GB or so). Even 16GB will fill up fast with lossless content, though of course it will depend on the number of tracks that you end up loading on the player. And of course the CHOICE of which lossless format to use will be a determining factor (compatibility) in the brand of player(s) you will have available to you.
The WAV format is the about as "non"-lossy as it comes, but it's also non-compressed; the resultant WAV rip will be essentially the same size as the original CD track. Other formats like AIFF and AU are also uncompressed as well. So-called lossless compression formats (FLAC, Apple Lossless, WMA Lossless, etc.) result in files roughly half the size as the uncompressed file but again still take up considerably more storage room than the typical MP3 file ripped at 128k.
While most DAPs worth their while will support WAV, relatively speaking not many devices can support compressed lossless. Apple Lossless is only supported on iPod devices, so you're stuck with the sonic limitations that comes with that direction. WMA Lossless can be used on Zunes (with the right updated firmware), several Sony Walkman series, among a few others. FLAC more or less only has Cowon and iRiver supporting it here in the U.S. market when it comes to portables; it has a greater following with dedicated home audio gear. The other uncompressed and compressed lossless formats have just as few if not fewer portable device options.
If you're truly after the 'best quality' option, then WAV or WMA Lossless is probably your best choice of audio file format. Assuming you'll go with the least restrictive (WAV) in terms of widespread device compatibility, off the bat I'd recommend any of the following devices currently available:
- Microsoft Zune 120 (battery charge--or lack thereof--is an ongoing issue with hard drive Zunes, however)
- Cowon iAudio6 or iAudio X5
- Creative Zen X-Fi 32GB (yes, there have been reports about the quality issues with the Zen X-Fi, but I've had mine 9 months and it's still working just like it did when new and besides, even iPods have had sporadic QC problems)
If you can live with 8 or 16GB, then a larger batch of DAP/PMP devices open up to you:
- Sony Walkman S-series (Sony Walkman devices by and large are TRULY great-sounding units, bar none)
- Cowon (lots of choices)
- Microsoft Zune 8 or 16
- Creative (lots of choices, though again you'll have to decide on how comfortable you'll be with the reported quality control issues)
Samsung has a new P3 coming out soon, but I'm not sure if that will support lossless in any format (their current great-sounding-but-lossy-only P2 doesn't). Sony also has their new X-series player coming out shortly. Again I can't crow enough about the sound quality of Sony Walkmans; they really are special-sounding units. That said, Cowon also makes really great-sounding DAPs and PMPs as well, as does Creative.