I have to say, the RTM version of the Mudic app is very good. Up until and including the Release Preview, I hated the music app and used Windows Media Player. Now, I use the Music app.
I'd be curious to know what improvements the RTM music app has over the RP version (which I'm using as I type this). Not to mention, since the app can be updated separately, it may be updated prior to GA.
But, like Subhadip above, I'm using the OneNote MX app (Currently plotting my Fantasy Football strategy), DIscourse (which has been pulled for an update), and Tweetro (snapped right now).
USA Today and News360 are excellent as well. The NBC News app (formerly MSNBC) is well laid out and excellently designed.
Digital Folio is a great product comparison app for us electronic and tech geeks. XE Currency (Conversion app), especially when used with the Search feature while shopping on the web for example.
And, if we're talking games, Armed! and Wordament are stellar. Although, Wordament is rather difficult to play with a track pad. With a mouse and touch, it's just fine.