Lemme think... gonna leave out the seasonal releases since they tend to be hard to find. Ditto for the original Master Artist series, I guess.
Hibiki's Master Artist 2 CD pairs her up with Haruka and Miki. Her songs, even the covers, are very good. Although her horrible anime episode kinda ruined Trial Dance for me. As a bonus... I can only guess at the content based on the sound... seems like Miki helps Hibiki overcome her reluctance to wear a bikini.
Master Special 1 (Haruka/Yayoi) and 6 (Miki/Iori) have the most upbeat, up-tempo songs. Downside is that the original tracks were plundered and reused on the anime series CDs. Speaking of the anime CDs, you can probably find previews of them floating around.
The 961 (Overmaster) CD has no talk tracks, just 3 songs, but holds up very well.
I can't speak for most of the Master Artist 2 CDs- I listened to bits and pieces of most of them and liked very few of them.
Your optimum choice probably depends on whether you prefer remixes (Live 4 U), rehashes (Master Artist and anime CDs), or covers (everything else).
Or you could hold onto your cash and wait for the next big thing.