Update:
I've earned enough tears to get me a second assistant skill. I'm now a budding chef, as well as alchemist. Yippee! congrats to me...
It is, however, more time consuming (and coin consuming) then just maintaining one skill. also, it tends to clog my quest list. so I guess I'll have to be careful which quests I take from my mentors. Coins can be bought from players at a ridiculous price, so I think I'd rather find them then buy them, at least until I can make a profit from my skills.
One day, I'll become rich selling those coins, myself lol

Yes, sometimes alc needs things crafted from chef. All three skills benefit each other.
I.E. you need colorless and odorless from chef in order to craft things from alc. You also need colorless in card master as well. There are many other things not listed but yeah, it is useful to have two assistant skills that help one another.

Well, I've already started on the path to become a renowned alchemist. But I guess when I have the time and money, I'll start on a chef career, as well.
At any rate, it seems like a good thing to have a double assistant skill, but it's more time consuming so I'll advance a bit as alchemist before I start a new skill.
Thanks for the input, though. much appreciated.
A question, though: are there skills that help each other more than others do? for example: Can a chemist craft things that benefit a card master, or a card master benefit a chef, etc.? I think it would be useful to have a pair of skills that aid one another the most.

You can always try leveling two assistant skills on one character True alchemy is helpful and is pretty easy to level, but it takes some battlefield points for some of the recipes.
Chef also comes in handy since you can sell some food items for a good amount of money and farming the mats aren't that bad.
As for card master, which is the most tedious to level imo, it also comes in handy as you can make your own cards without wasting around 10mil+ on cards made by others. The mats are a pain to obtain but it's worth it if you're willing to give it a go.

Just try it out, there's no restrictions
As for that tutorial idea, I think that's a good way to help new players learn what assistant skill they would like to have.

Good luck!

Thank you all for your inputs. Alchemy it is, then.
I'd like to keep this discussion alive, though, because I think it's a big subject and probably many people can benefit. If someone can pick up the glove and make a short tutorial on how to select an assistant skill, that would be awesome.
I think such a tutorial should include what was said here, including maybe a discussion on which is the best for each profession or game style.
Maybe there is such a tutorial, hiding within the forums or somewhere else, that I don't know about? If so, a reference would be nice....

As people above have said, Alchemist is a good choice for your first assistant skill. It's very easy to level up, especially since all of the materials needed can be bought from NPCs.

Well, if you wanna make money, go with alchemy and craft the flawless enhance cards. I don't sell them myself but I've seen the flawless red enhance cards for around 800k - 1m each.

I think alchemy is easier than card master because it requires a lot less ingredients. All you have to do is decompose pretty much. I think the flawless stones are strictly decomposed from level 60+ gears. Then level 80+ gears give higher tier stones.

I'd suggest you get all the assistant skills eventually though. I've personally maxed alchemy and card master, and I'm currently working on chef. I got them in different characters.

Thanks Spunny. This helped a lot.
I'm starting to lean towards alchemy. I think ingredients for food items are easier to find, but I can always sell them, right?
Besides, I hate to cook (in real life. so much easier on the game, isn't it?)
Still waiting for feedback on this. I will probably decide tomorrow.
Thanks for the help, please feel free to add to the discussion. even after I decide, this might help other people.
Isn't community a beautiful thing?

Alchemy is mainly necessary because of untradeable abs pots. Even if you're not that into pk/bf, it's still very helpful to have the 50% abs pots in dungeons if you are tanking mobs as a precaution. So yes, I would also recommend this. Even if you don't want to do it now, you might pick it as second skill

If you are still leveling, I would recommend chef over cardmaster. Simply because the most useful cards to put in gear would be the ones you need to be higher level to create. Of course, the same goes for food, but I believe there is a level restriction on higher food, whereas for cards, if you ask a higher level character to help you slot it into your gears, that works too. Plus you can buy the lower level cards for fairly cheap prices from Nojacque in Terminus.

Btw, from what i recall you can drop an assistant skill (if you really regret it) and learn some other skill. But then all your previous effort is wasted.

Thanks Adam, it's very helpful.
I can't invest enough in another characterat this stage, not enough to develope an assistant skill. I'm not sure I like battlefield. My last (long ago) experiances there were not so good: Half the time I've been runnung around in the wrong direction... looked very chaotic to me. But I'll try again and see if things became more... ah... managable on this version.
In the meentime, I'd really like to have a few more comments or recomandations, and I'd apreciate any contribution to this discusion. I must give it some serious thought...
I'm leaning towards chef, only it sounds so... ORDINARY.

For CardMaster, this is the best assistant skill if you want to make gold, however the mats for the cards might be hard to get for your level, Not to mention, this is the longest, and hardest skill to level up. Chef and Alchemy are pretty easy. so I'd recommend this for later on. My recommendation is to have an alternate character with this assistant skill. There's even some nice people in game that will make you cards for free, if you provide your own materials.

For Chef, it's a nice skill to have considering there are some nice foods to make that can boost your stats, and really easy to lvl it up while you lvl your character as well.

For Alchemy, I think it's a best first assistant skill to get, considering some of the pots you make are untradeable, which are essential if you plan to join Battlefields.

So in order for my recommendation is, 1) Alchemy 2) Chef 3) Cardmaster