Adult Electric "Eel"(knifefish)wins IMO. Whereas a GATF can deal serious physical damage and has size. An electric eel can straight up murder you without even touching you. These things can grow up to 6ft and weigh in at 45 pounds. Not very impressive right? That is until it unleashes 500 volts at 1 Amp on you. While this won't kill most of the time, it can hurt us considerably. To a fish, this is more than enough to stun/kill. Oh and in case you didn't die from the first shock, the fish can fire another one...and another until it's out of juice.
The scenario would be as follows: For the Electric Eel vs GATF(or any fish for that matter)
Big tank, open space. Both fish are released in at the same time.
The Eel finds a corner and hides. The GATF locks in and torpedoes at the eel...only to suddenly jump and fall over? Bam.
Vicious? No. Meanest? Nah. Baddest? Damn straight.