T3-M4 may be great with computer, security, and repairing, but he is just an astromech droid. HK-47 on the other hand, is both an assassin droid and a protocol droid. HK-47's skills in translating is as good as C3-PO's, but his conversations are always so violent. His skills in assassination are even better since he has assassinated more than 100 people, probably 1,000 or more. HK-47 is definately my favorite and always will be, as you can tell from my avatar. By the way, you should have put a choice to choose both as your favorite, as I don't hate T3-M4. I always think of HK-47 and T3-M4 as an evil version of C3-PO and R2-D2.
There is no emotion; there is peace.
There is no ignorance; there is knowledge.
There is no passion; there is serenity.
There is no death; there is the Force.