Flipper first, hole next, then thumbstuds, but never nail nicks. From a production standpoint i think its much easier to make a flipper or a hole (less material used less processing too) which to me is a bonus, at some point id like to manufacture blades and the less i need to touch it the better (lower costs for everyone!) All this aside the flipper is just really satisfying, especially when there is no spring and it just opens with a smooth action. The hole works great too because it has a wonderfull tactile feeling to it, plus it can be opened slowly or quickly. When it comes to a thumbstud ive noticed it can make the blade pop out really well but its at the expense of thumb nerves being slowly torn off. Lastly talking about the desruction of fingers nail nicks are the worst; i can remember being a kid using a nail nick with a partucularly sticky knife, which is a wonderfully painful experience.
One more side note, flippers are easy to produce but they require the material to be extra thick which means you are more than likely going to waste precious blade steel, so from a "i dont feel like wasting steel standpoint" thumb holes are where its at. From a "ohhh thats nice" standpoint flippers are where its at.
OH and super last note i want to build/buy a knife with a really loud and satisfying CLACK as it locks, what would best acheive this (type of lock/materials)