I have three shistura manherti's- the red tail tiger loach- different from the previously mentioned tiger loach- they don't get huge, but they are so bad I've taken to calling them the SH*T's . They attack like a pack of dogs or something- ate all my shrimp and guppies, and have killed anything that isn't smart enough to stay out of their way.
Other than that, the most aggressive is by far the tiger loach and the other types similar- the Queen's and the berdmorei. Skunks and red-tail are close seconds, at least to other loaches and other bottom dwelling fish.