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Back in 2010 I downloaded a torrent with "all the Vocaloid albums released up to August".
It had about 300 albums.
I still haven't finished listening to all of them, but it was a good way to introduce me to a lot of good stuff.
Perhaps the best ones on a surface level were the compilation ones, "Exit Tunes presents the Best of Megurine Luka" and things like that.
Once you get these compilation albums and some artists draw your attention, you can go for their works, picking some from each point in their career.
My recommendations would be, partly because I like some, partly because they are popular and relevant:
Kikuo, MitchieM, DECO27
DeadBallP, 40mP.
If you don't care about everything being old, I can recommend some albums.
Or I can give you isolated tracks.
Another big entrance to Vocaloid for me was Osu!, rhythm games were full of Vocaloid and Touhou songs to get you initiated. And Vocaloid also has Project Diva dedicated to it.
Finally, there is the vocaloid thread on /jp/ where you could ask for stuff, I'm sure they'd also have charts.