I have a few, though they're not crazy holy grail type instruments. They're mostly ones that slipped through my fingers because of circumstances. I don't know how much I would have actually liked them, but they stick out in my mind as missed opportunities.
Some years ago, Mim had a couple cedar/myrtlewood Breedlove tenors that were apparently prototypes. I'm not sure what they were prototypes for, since they never went into production. The current Breedlove production ukes are Chinese laminates with solid tops. They're actually great ukes for what they are, but I wish I could have gotten a hold of one of the prototypes Mim had.
A couple years ago, Hodge/Doc J put up a silver oak Larrivee baritone in the marketplace. I was recently able to track down a Larrivee baritone that I'm very happy with, but I wish he'd held onto that silver oak one until this year when I started looking for one.
Just last December someone put up a sunburst Kinnard tenor in the marketplace. I think it was up for almost two weeks. I was sorely tempted, but at the time, I didn't think I wanted a tenor, and all mahogany isn't usually my first choice of tonewoods. Still, I'm a sucker for a perty sunburst.
I think earlier this year, DJ Morgan in the UK made a purpleheart mini pineapple, but he wasn't shipping to the States at the time.
More recently, right before everything shut down around here, I was in a shop playing a KoAloha anniversary silver. I think it was tenor #003, and it was really beautiful to look at. But I wasn't sure about the sound, so I went home to think about it. Later, I got in touch with them and asked about it, but they'd sold it.
I'm still casually looking for a KTM-25. I might get one if I find one I like, but I'm okay if it doesn't happen. I found a great Larrivee baritone, so I'm not so curious about the silver oak one, and while a mini-pineapple would be kinda neat, I'm not searching for one. If the Kinnard showed up again, I'd probably end up hesitating until it slipped by again. But for some reason, I think I'm probably most interested in what happened to those two prototype Breedloves.