
LEON: The first Vocaloid voice I fell in love with. Something about his tone and slight "charm" to the voice was interesting to me. I personally enjoyed using older technology, as if to say old technology can still be used in beneficial ways. He was the only voicebank I owned for a good while, but I enjoyed learning more about the Vocaloid Editor that way. It's interesting when V1 is the first Editor you use - you can see how Vocaloid has grown from it, but surely it's not the same without those Resonance parameters the first editor had.
LEON served as a gateway for me into using Vocaloid. I messed around a lot with his voice to get an idea of how I could use his voice; it made me enjoy him even more.

LOLA: Now, a few years back, Zero-G had some kind of sale going on, before they announced they were going to retire the V1 voicebanks from their store, that being LEON, LOLA, and MIRIAM.
I can't recall how long it was ago before the announcement, maybe a year, but there was also a bonus: when I bought LOLA, I also got sound samples. I was excited! Another Vocaloid to my collection, and I'm glad I purchased her. I became more attached to her voice. Like LEON, I messed around to get an idea of what her voice sounded like. At the time, I didn't know I'd use her voice years later to help create the song Invisible Being. I love using the Vocaloid1 Editor, and I wish I did so more.

Hatsune Miku (V2): Now, let's get to the Miku part. At this point, I was much busier with things like school, so I didn't use Vocaloid as much unfortunately. But, I still made it a goal to work on music, specifically for the future.
There was a good auction going on at the time, so I decided now would be the time to finally pick her up.
Like with LEON, I wanted to use "old technology", as that's also what some of the most well-known producers first used: V2. I believe V3 was already out at the time.
And I was excited (again w, I'm gonna use that word a lot). I absolutely love Miku, and enjoy her voice. I can't describe it, it has some sort of cute, robotic-like charm to it - you know it's not real, but it's better that way. She's here for you, she doesn't need to be real.

Hatsune MIku (Append): A bit of a similar story, but I absolutely love Miku's Dark Append, and knew I wanted to use it for potential future projects. I picked it up and began messing around a bit with the voices, and realized each one of them could be used in effective ways. Despite being the same character, each voice felt unique, as if bringing a different personality. I find "personality" can be a beneficial thing to bring into songs, so I'm glad to discover each voice could be used well.

Yuzuki Yukari V4 (Jun and Onn): Yukari's Natural and Whisper voicebank. Yukari was on a major sale at the time, and I began considering picking up another Vocaloid. Upon consideration, I realized how effective Yukari's vocals are in some songs - especially ones that interested me. She's a Vocaloid that's well known and is up in popularity and, is certainly, a gamer like me. I thought her natural vocals could be used well, but upon hearing more about her whisper voicebank, I fell in love. It reminded me of Miku's Dark append, and I decided it would be a good investment, especially considering the sale.
As with the other voices, upon use I've gotten to "know her more" and love her voice even more so. She even comes with some exVoice files (a lot!), that made me happier with my purchase.

Yuki Kaai V4: I purchased her alongside Yukari Jun and Onn. She was on sale, and for an amazing price too. I put some thought to this, as I had also thought before of picking up Yuki. Her childish, innocent vocals could make for something intriguing, I believe. Perhaps (and hopefully) unique.
After much thought, I decided to purchase her voicebank. It could very well be the last time she was on sale after all (I don't think it was, but I still think I got a good price either way).
I'm eager to see what I could create with Yuki's voice! Her childish voice could make for some heartwarming music. I don't have access to something like a kid's choir, so Yuki may be the way to go then!

Dex: Ah, Dex. This purchase was for Christmas - on Christmas day even! He's a Christmas gift for me. I picked him up just last year, again at a time where projects were rather busy.
But I had been considering Dex for a while at the time. It would be another English vocal, specifically one for a newer engine even, and also give me another male voice. On top of that, I was convinced Dex's vocals could work pretty good with rock, so I thought he would be a good voicebank to add to my collection.
I wish I could use him more, it would help me enjoy him even more, I already know. Hopefully as projects die down and I have more music-related ones, I can really get a good grasp of his voice and love him even more than I do now. But, I'm happy with Dex, I'm glad I got him. I'm eager to see what I might create with Dex too. I was using his voice I think a week ago, and I began getting an idea of some of the potentially unique ways I could use him
Bonus:

Megpoid Talk: I believe Megpoid Talk was on sale when I picked her up. I thought it would be neat to pick up a talking synthesizer, and this one even came with two other generic voices - a basic male and female. (I got a funny idea that these voices actually ARE Gumi, she's just good at imitating.)
I became interested in getting talk synthesizers thanks to watching Let's Plays. I thought Megpoid Talk could create a VSQ from what you created, and I honestly just figured it out - I thought this would help with creating talk videos.
My drawback though is it seems like Megpoid has quite a bit of licensing tied in to her, and I don't know if that even includes just using a VSQ you made with the software.