Thinking about it, probably the V6 starters? 99% of the time I forget that they ever exist, likely because they are almost never talked about or used. But from what I've heard, they're actually pretty high quality, especially for V6 banks. The best use of them I've heard is probably
Self Mind Control by EZFG (rest in peace) with Akito.
In terms of SynthV, Mo Xu comes to mind for me. Her voice is beautiful, high quality and extremely versatile, but she doesn't see much use probably due to her status as one of the three SynthV2 starters. She's going up against Mai, a returning favorite from SynthV1 and Liam, a very good masculine voice that's also quite versatile. And add in the fact that her native language is Chinese meaning that most people will almost certainly never use her for her recorded language, and that just creates a loop of unnoticeability,