I'd say Tactical because you actually get ground combat training that raises your DPS.
Science may be a close second, though. The holds and especially the root are very handy. I have an alt Gorn science officer I made for melee and it works pretty well. Gorn strength is a nice bonus too, I think.
Engineer...eh. Weapon malfunction is very nice for this and dropping mines to snare would probably work but I feel like the Tac +DPS abilities and the Science ranged roots are probably better/more practical.
Incidentally, I think it's possible that martial arts (barehanded combat) is better than bat'leth combat. I was just noticing today that left punch does an integrated "interrupt" and right punch does an integrated "-100% run speed for 1 second" (which basically means they can't sprint). So when you're in someone's face, punching them and doing combos, you can also be interrupting them and negating their ability to sprint away. The bat'leth doesn't do that for you.
But yeah, to echo what the other posters have said, melee in PvP is mostly just silly. I bust it out when we're grossly beating the other team but it's nowhere near the DPS of carrying two good weapons to switch between.
In PvE I understand it can have its uses against NPC engineers and borg that may have strong shields and weak hit points but for PvP it's just not a practical option after T2. And even in T2 it's pretty dubious.
And I doubt there are many people who have tried harder than I have to make melee work in PvP. I know the combos, I'm good at sticking to people, I have my melee combat fully trained, I have ground combat traits, but it's still better to just shoot people.
If they ever put weapon special attacks on a shared cooldown (which I think they should do) then you MIGHT see melee make a comeback.
The biggest issue I have with melee, is that if the enemy player just takes half-a-step backwards it nullifies the combo I was building. No combo means no exploit/expose attacks. They really need to add some sort of range to melee combat to make it viable in general (even NPCs tend to back peddle.)
Oh yeah, palm strike is so over-powered it is ridiculous. It stops you dead in your tracks for several seconds. wtf?
Tac gets a bonus to damage and some other abilities like leg sweep which can help. Still the weapon is pointless but iI believe this is correct who would really use a sword in a battle where everyone else has guns.
The damage may not scale but I still use it in Tier 3 (its quite nice being the only one :p)
I currently use Lunge, Leg sweep, Motion acceleration and stun grenade (basically any close combat tactical kit is what your after). Also I use the physical augmentation armour that gives a +35 to total health. I was going to take the health regen but it seemed too small a number to be worth it. I may try it out later though.
The Bat'leth currently due to be highly ineffective (admittedly) is very hard to use. However getting kills with it regardless to how weak it is feels great. Its all about timing currently. My usual tactic is to try and ambush them somewhere where they dont have the time to run away. If they start backing off then you use a lunge attack at them, followed by the knockback shove and then pull off a 1,2,2 combo which lands a DoT. Once that is applied hit then twice more then use your leg sweep. Pull another combo off which can be anything really and then finish it with another knockback. You will probably be about 3/4 of their health down by this point and only really need to do 1 more combo.
Basically going by that you just need to keep them on their butt for as long as you can. If they are a science officer then all they need is 1 second to heal and your screwed. Engineers have their annoying tactics too which can really screw you.
Hopefully they will fix the melee up though and it wont be so useless. Including Martial Arts (would like to see this take more of a serious look than its current laughable state). Would also be nice if there were variations to weapons instead of just the Bat'leth (as godly as a Bat'leth is Q'APLA!)