This was my first build but it's the one I like best! Keeb.io Iris Rev6 Hotswap
Acrylic plates (white + frosted middle)
KAT Milkshake Weirdos keycaps
Kailh Box Whites
If I have to pick something, I'd say the thumb clusters. I barely use the top ones in the center because they're a bit of an awkward reach for my hand size. The bottom ones on the outside are also weird because you'd need to do some finger curl or move your hands. I just ended up mapping the keys I don't use as much over there and it hasn't been an issue, but it is some wasted potential :)
The Iris is my favorite keyboard. I have built 3 of them so far. I agree about the top thumb cluster switches, but I put encoders on mine. I still don’t use them much, but they are “neat”. The bottom outside thumb cluster are extremely comfortable for my hands (it’s where my thumbs naturally rest) and are some of my most used keys (enter and fn). Space is my middle thumb cluster which is also great. The inner most ones are the hardest to reach, but are reserved for alt and win.