I'd have to say shells, particularly the "shells & cheese" size. I always have quite a few shells stick together and end up undercooked, and I don't really encounter that challenge with other shapes.
I actually like orzo a lot, but I've always had it in dishes where it behaves like (and is possibly mixed with) rice. I think it adds a nice (creamy?) balance to some other carby things, such as a veggies. Trader Joe's sells one that really like that has orzo mixed with spinach, sundried tomatoes, and feta(?) cheese.
i use shells a lot, even for bowls of just pasta and sauce (vs a plate of sauce over spaghetti noodles). it's just easier to scoop 'em up with a spoon.
use plenty of water and stir the pot frequently. i only have a problem with them sticking together while cooking if i neglect to do those two things.
they're great in pasta salads or mac & cheese when you're using peas in whatever you're making. some of the peas work themselves into the shells. it's like they were made for each other.