Cybertech is the only one of those three that will have anything unique in the Endgame after 1.3 hits. Reusable Grenades can be quite useful, although not for a Merc as much as for some other classes (you already have ranged cc and aoe.) You can get a few items slightly quicker/easier with the other two but now people will be able to augment everything the USP of augmented relics, bracers, etc., is no longer there.
This is not entriely accurate. Patch 1.3 will give the ability to add an augment slot to any armor/weapon, but in order to add this augment slot it will require a kit that can only be made by crafters. Augments will still only be able to be made by crafters as well, so any of these crew skills will be useful as they will offer the augments and augment slot kits for their particular gear type.
My personal suggestion is to go with armormech since it will give you the most ability to customize your look and the majority of what people need will be armor (more slots per character than implants or weapons). Ultimately, the majority of people go biochem on their main for the stims/medpacs that are the highest level, especially if you plan to PVP. My crew skill suggestion is to level through armormech first to craft Rakata bracers and belt, then drop it for biochem and pick up the other crafting skills on alts if you want them. The armor, weapons, etc. can be used by any of your toons (although the Rakata gear is bind on pickup, which is why I suggest doing it first), but the biochem craftables require a skill level of 400 in biochem to use. So you can continue using armor you've crafted that is BoP and buy other armor other places or craft on alts, biochem crafts can't be used if you drop biochem.
If you simply want to do the best crafting possible, there are multiple guides you can google that will tell you how much of each mat you need to craft at the fastest pace. Essentially, you will want to keep making the highest level green schematic you can make to level your crafting skill to the max level and spend the least amount of money. If not, then whatever crafting skill you pick will be equally rewarding as you level your toon.