I recorded a handfull of UTAU voicebanks in the heyday of my vocal synth era. I could barely understand how UTAU worked, though, so they were only Japanese CVs and never really saw the light of day. Maybe I'll try to work with them in the future.
UTAU is a free vocal synthesis software developed by Ameya/Ayame. This software became popular for those who couldn't afford VOCALOID, since piracy was extremely frowned upon around the time of its release. You can make and configure your own voicebank, and there are multiple different ways to do it. The software also comes with its own voice, Utane Uta (Defoko), created with AquesTone. The last update to the software was in 2013, but it's still widely used.
In 2021, OpenUTAU was released, working with the UTAU engine but adding many quality of life improvements. While it remains compatible with all UTAU voicebanks, it has an appearance closer to that of a commercial synthesizer. It has 'phonemisers' that aim to make inputting words and phonemes easier than it was in the original program, and supports a variety of voicebank types. It also has multiple singer capability, which the original UTAU lacks. This version of UTAU was created by StAkira.
That is entirely up to you! I would consider it a great tool if you want to learn how to use a vocal synthesizer. It's free, and the variety of voicebanks (and their configurations) feels very similar to VOCALOID, and likely other commercial synthesizers. If you want to learn how to tune, OpenUTAU is definitely a good choice, since by default it has no AI tuning option.
You also have a variety of voicebanks to choose from, since anyone can make an UTAU. There are a wide variety of languages as well (though OpenUTAU only provides phonemeiser support for a few—vanilla UTAU does not have this feature at all). Additionally, if you dislike one type of English voicebank, there are other kinds to choose from. This is true with a couple of languages, but not all.
Не могу я жить без тебя
Bank: M1NT0, Orig: А. Апина
Вологда
Bank: AICA, Orig: Песняры