I've been a Vocaloid fan since 2015, so I got to see Luka, the Kagamines, and Miku get their V4 releases. However, I didn't get into
using vocalsynths until around 2018 (I cut my teeth on RenoidPlayer and Sinsy and eventually was able to move into UTAU and SynthV once I had access to a PC).

I got my first Vocaloid in June 2021, and that was the Hatsune Miku V4X Bundle. Many say she's expensive for what you get but I'm really happy with her AND the feature-rich DAW she comes with (didn't use to be so feature-rich, thank you PreSonus for Studio One 5). I'll post my recommendations on each of her voicebanks:
Original: Super smooth-operating and easy-to-use, handles pitch editing REALLY well. Very bright and clear, pronunciation isn't as wonky as in V2 or V3 (some prefer the odd accent though, e.g. how she pronounces "ki" like "kei" and "mi" like "mei"). Unsurprisingly handles EVEC the best. Very much recommend.
Dark: Not quite as breathy as V2 or V3 dark; vowels are clearer. Nice mature airy tone, can be very expressive. Not quite as user-friendly as Original, Soft or Solid (mostly due to lacking EVEC; not bad to use overall) but has a more realistic and emotional tone. I recommend if you can use her in the right context. Her tone is super pretty but EVEC is very tempting.
Sweet: VERY VERY quiet. PLEASE boost the volume of this girl. Adorable and innocent tone, fairly similar to Kaai Yuki. One of the further voicebanks from Original in tone. I recommend if you don't mind having to boost her a bunch in a mix.
Soft: Actually has a normal volume, unlike Sweet. Gentle, mellow and ladylike tone; similar to Dark she can be made to sound very emotional. Closest Append to Original in tone, and similarly easy to operate, partly due to both of them and Solid all having EVEC features.
Solid: I've historically thought her tone obnoxious, but I'm warming up to it. A bit nasally, with open vowels and an overall clear and powerful sound. Fairly similar to Original with a deeper timbre. I've become more fond of her upon realizing she's a LOT of fun to use- also FAR from the pain to mix Sweet is (xP). Seriously, she's a blast. Handles EVEC and growl really well. Recommend.
English: For a JP-EN voicebank, she is fairly easy to use, and has a bright, cheery tone with a decent amount of power to it. She's close in tone to Original, maybe a bit huskier. Her accent is very noticeable though, but she doesn't really make an effort to hide it, unlike V3. Handles L sounds better and has clearer and less wonky, though also less English-adjacent, vowels. Handles growl really nicely. Probably the best Japanese-accented English voicebank I know for beginners, besides maybe Fukase or Kaito. (GUMI is known to have great pronunciation, but her voicebank is also very wonky, notorious for broken transitions and would probably cause a lot of frustration to a new producer.)

My second Vocaloid was AVANNA, whom I bought on my birthday that same year, Aug 17 2021. I don't use her NEARLY as much as Miku, but her tone is super pleasant as is her Irish accent. I do tend to find her voice quite deep so I often use gender factor on her. A native-accented voice like her is refreshing when you're like me and work with non-native-accented voicebanks a lot.

My third and final Vocaloid was Hatsune Miku V3 English, whom I bought secondhand that same month. I know she's not V4, but I'm very fond of her and recommend her to experienced producers (you can't buy her standalone firsthand anymore though). She has a very pleasant low, husky tone, doesn't have that "V4 garble" an above user mentioned (she and AVANNA both have a cool "V3 twang" going on though), and though her accent is notoriously strong (and her tone can be a bit nasally or "froggy" like Kaito), it can also sound pleasant- I especially like the way she pronounces certain consonants; some vowel combinations are nice while others can sound warped and wonky and she has a tendency toward an intrusive-R habit (which V4 almost got rid of, I think it lingers from time to time). Sounds less "flat" than Miku V4 and can be quite expressive; her vowel sounds are also closer to American English than V4 which at times (at least for an American like me) can be beneficial. Of course, she is notorious for her accent on L-related sounds and I can confirm that there are struggles. I strongly recommend her to experienced producers because she can produce nice results in the right environment, but not to beginners who will probably struggle using her.