gumism - Look at dat website. Somehow, the Japanese are particularly gifted in making very modern, flawless websites. All the sound clips sounded really high quality. *shivers* Plus, I really don't think you can go wrong with Megumi Nakajima's voice, even when it's technoized.
Symphony - ohhhh those clear voices... so perfect....
Vocalonexus - I think this one may be my favorite. It has just the perfect mix between all the different vocaloids and contains many of the popular songs like Matryoshka. If I had to pick one to buy, I would almost definitely pick this one. AND OH GOD LOOK AT THAT PRETTY INTRO TO THE SITE. *0*
Evil's Kingdom - I think it's just Rin and Len on this one which is cool and all, but don't really like them as much as the other Vocaloids... Meltdown is an awesome song, though.
vocarhythm - Whenever you roll over the side buttons, Miku makes a "pon" noise. It sounds kind of funny. xD You can't really go wrong with Miku, though some of the songs on the "menu" didn't really suit my tastes.
I'll be doing some other Vocaloid related posts in the future such as:
-Top 5 Vocaloids
-Top 5/10 Vocaloid Songs
-Luka x Gakupo (the only couple I really like out of all the fanpairings)
-Why people like Vocaloid