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.