After having tried them all, I would say (in order of most useful / pays off best)
- Cybertech / Artifice : most useful across toons. You can make armoring, mods, enhancement and crystals. Also, most of what you craft sells very well. You also make droid parts, speeders and spaceship parts.
- Biochem for adrenals and implants. Good sellers as well. You can also make medpacs.
- Armstech for barrels, blasters and rifles.
The others have been marginally useful. Armortech and Synthweaving are ok but with so much armor dropping from the quests and missions there isn't much special you can produce.
I would almost agree with this, but would add some explanation.
I have a 400 level crafter for each of the skills (Cybertch, Artifice, Armstech, Armormech, Synthweaving, Biochem).
1. The one I use/used the most both for leveling up initially and for gearing up alts is Cybertech, hands down. It allows you to craft for yourself 2/3 slots on Orange Armors and a slot on Orange Weapons/Offhands. The armorings and mods available from planetary comm vendors are almost always the higher endurance versions (Guardian, Force Wielder, Commando, Patron). While the others are available, level-wise, they seemed to be behind. So, on Drommund Kaas for example, the comm vendor sells Guardian Armoring 4 and 5, but only Might Armoring 4. Same with mods. So, Cybertech allowed me to keep my toons' orange gear updated. Which also allows you to spend the planetary comms on other things, which is nice. And as has already been pointed out, the things you can make with Cybertech generally sell well on the GTN.
2. Artifice was the second best for leveling up my main and alts, as it allowed me to craft items to fill out the remaining slots on all Orange Armor/Weapons/Offhands. It also sells quite well on the GTN.
3. Armstech was helpful for the Barrels I could make, and is helpful at Endgame for being able to craft Augments and Augment Kits.
4. Armormech and Synthweaving are generally the same. They both craft Armor (including Orange Armors) that can be used by alts/companions or sold on the GTN. Similarly, it's helpful at endgame for the Augments and Augment kits, as well as the Rakata level belts/bracers that are Bind on Pickup that can be made. So, by no means are these skills worthless.
5. Biochem is the one I used the least while leveling up. I always seemed to have plenty of medpacks from just looting mobs. However, it's certainly helpful since you can make your own medpacks (of better quality) and stims as well as implants. And the Rakata level medpacks and stims are wonderful for endgame. Plus, being able to make Exotech stims for the GTN or a guild/friends is extremely helpful at endgame as well.
TLDR: All the skills are viable and decent. For leveling up both a main and alts, Cybertech and Artifice seemed to be the most useful. For endgame utility, I think Biochem is the most useful. For moneymaking and help both leveling and for a jumpstart on endgame gear, Armormech/Synthweaving are fantastic. It all just depends on what you want/need and what class you're playing.