If you're in the market for a vocal synthesizer that'll work with most digital audio workstations (DAWs), Emvoice is a great option. It offers photorealistic vocal synthesis in VST, AU and AAX formats, so it'll work with most DAWs. Emvoice is geared for high-end, realistic vocals for music production and audio post-production.
Another contender is Kits. This collection of AI audio tools includes tools for cloning voices, generating singing voices, removing vocals and mastering audio. Kits is designed to make music creation easier and more creative, with a variety of tools that can be dropped into your workflow without requiring a bunch of plugins or software purchases.
If you're looking for a wider range of music production tools, Orb Plugins offers a range of AI-powered plugins, including Orb Synth X, a wavetable synthesizer. It's not focused on vocal synthesis, but it's got a deep sound design experience and works with most DAWs, so it's a good option for producers and sound designers.