There's a couple ways of looking at this, but the market-share of players supporting AAC over Ogg is ridiculously lop-sided as is online music sales... towards AAC. This is of course because of the iPod and iTMS (add Real if you want). As for being locked in, the iPod isn't the only player supporting AAC (to prove a point, but the other players are minor ones), though it's the only HD one (well two), off the top of my head. There's no reason why (AAC is not owned or created by Apple) it won't or can't be supported by more players in the future. Remember until the Karma came out, there was only one major company supporting Ogg with HD based players, iRiver. Now there are two (unless you want to count much smaller companies).
As for WMA, if WMA Pro ever gets implemented, that would be great, but otherwise its space/sound compression is worse than decent MP3 implementations (mainly LAME). So in reality the online stores (128-160 WMA standard) are generally considered worse sounding than iTMS (128 AAC) or Real (192 AAC). There are currently no Ogg online stores.
Not trying to push the iPod or AAC, but I think those things need to be mentioned.
PsyvariarDC2, it's very difficult to recommend one player over the other without knowing for info from you on what you're looking for.
For greatest ease of use (GUI and iTunes connection) and greatest compatibility with what's on the market now (third party support), go with the iPod (BTW, it has a low "failure rate", but this is a drive spin noise issue you may want to research here- my first made noise when used with Etys and Etys only).
For most customizable sound and one of the best price/performance ratios, go with the Karma. There is that hard disk "failure rate", but do some research and the issue may not be as big as some sound. Like the iPod, it also supports lossless compression. Again if I was on a Windows machine, this would be my starting point.
If you're willing to pay a premium (same price as iPod), and want some advanced features like record and digital line-out (read up on its quality - there's some debate), want playback organization options, and also USB recognized mounting, this is your player.
Then the Dell DJ, Creative Zens and especially NJB, and a host of other players are great other options depending on your needs.