(Spoiler I will name the companions)

Slot 1: Síora -> she gives you +1 vigor when friendly and is overall useful, also helps you avoid a lot of native tension, she is pretty much a fixed position

Slot 2: Vasco or Petrus -> When it comes to combat and exploration either of them gives you a free point in essence. You always want charisma 3 for the follower bonus and the dialogue boost. Not to mention the merchant discount, but that is not exploration related. And you always want intuition 3 as it makes gathering a breeze and nets you an INSANE amount of extra loot. In essence both character option saves you a point. Vasco boost can be activated basically at the start of the game, Petrus is a bit slower. I suggest starting with Vasco and if you like Petrus swapping them when he becomes friendly.

Despite they are both good characters as well Kurt and Aphra are better on the bench for a purely metagaming perspective. Kurt is the weakest companion in my opinion as he is geared to be a tank, but doesn't draw attention. Aphra deals a ton of damage and is more useful, but she dies to a breeze so requires constant babysitting and if you are like me who likes to have my companions to be useful on their own she is meh. Both of these character is super useful to swap to in camp however as both of them give you crafting when becoming friendly essentially saving you two points. Again if you like their personality or you are curious of their dialogues from an RP perspective it is understandable, but if you take either you are putting yourself at a mechanical disadvantage (on higher difficulties it matters more).

Good news is you will have them all at max before half the game if you are a completionist.

Charisma: Level this to 2 as fast as you can. Your cape always give you a charisma, there is literally no point taking the last point. If you prefer Petrus as a companion I suggest using a memory crystal when he becomes friendly and lowering it to 1.

Intuition: As described above level this to 2 as soon as you can get it. It helps a great deal with exploration and conversations too. If you are not playing with a friendly Vasco for whatever reason then you will need to put the last point in, otherwise he will give you the sweet last free point.

Science: If you are technician its a must, otherwise you can pick it up later. That being said Science 3 and Charisma 3 is an insane money making tool. You can make dirt cheap infinite major healing poition, greater resusciation powder that is needed for all. It makes ammo very trivial. Furthermore technicians need alchemical preparations, if you are not one you can sell them for good money. Another pool is you can make greater swiftness potions for example (I think they are pretty useless due to their short duration) and greater antidote for example for two other pools and sell them for godlike amount of money even if you are a tech. Now as how to level it -> you can make a +1 science chest with the blacksmith, I recommend picking a lightweight one. You then should put two points in science until you get Afra and become friendly with her. With that you can respec for 1 points (the chest and her will give you +2 putting you to the desired 3).

Vigor: The only reasonable chest option to keep on yourself for all time is the +1 vigor one. Síora when she becomes friendly will give you +1 and you pretty much should keep her in the party at all times (unless you hate her or you RP). It means you should put a single point in it to push it to three eventually. I think it is NOT a priority however, as you will get by with the freebies.

Craftsmanship: Early on you will find several blacksmith gloves. Keep one. Don't worry if you sold it sometimes it pops up at the merchants. It gives you a craftsmanship. When you get friendly with Kurt it will give you another free point. This means you only ever want to put one point into this one and still have craftsmanship 3. It will save you a little gold and travel time as you don't have to use the npc who have access to it at start in New Sérene. Also after you get it to 3 you should always recycle trash loot items that have customized upgrades in them. I think it is extremely low priority however.

Lockpicking: I simply think this is trash. If you have all the other talents on 3 with the free points then you start investing in it. Also make a chest piece that give you +1 on lockpicking so you only need to raise it to two.

Overall my recommendation:
1. Charisma 3 (2 points +1 cape)
2. Intuition 3 (2 points +1 Vasco)
3. Science 3 (1 points +1 Aphra +1 Armor)
4. Vigor 3 (1 points +1 Síora, +1 Armor)
5. Craftsmanship 3 (1 points +1 Kurt +1 Gloves)
6. Lockpicking 3 (2 points +1 Armor)

Total to max out: At level 33 you will get the 9 points to effectively max out all talents with optimizing. If you forego lockpicking altogether as it is garbage you will max out all recommended talents at lvl 25. After either you can level them at your convenience. Since the armor swapping are for crafting and opening locks I usually pick the companion talents so I have a bit more freedom to swap them around. If you do side quest and exploration over main story then it also saves you the respec potentially.

This one is pretty straightforward though you have to make choices here as you cannot have them all and they are impactful. There are several way to go about it, but it will largely depend on your playstyle. I can give a few build options as this is essentially your "class".

Technician:
Max out accuracy to 5 -> Then Max out Agility or Endurance and put the leftovers to either.

I think this is the most OP as far as builds go as you can forgo anything else, but accuracy at start. Your main damage source is the gun, alchemical preparation tossing and fury bomb. If you don't care about melee and just want to shoot everything then go for Endurance first and then pick whatever. I sometimes preferred to melee a bit with a twohander hence my pick, but the bombs and the gun is enough for literally everything in the game and the ammo generation is trivial (I had about a thousand by lvl 20).

Warrior:
Dependant on two attributes: Level Endurance and Strength OR Agility together. I recommend leveling the weapon attribute first then endurance, to keep the latter on the level (or one level above) the armor tier you can find. Anything extra should go to Wisdom or Accuracy depending on your skill choices.

Mage:
Sadly you are dependant on two attributes here as well. Level Mental Power and Wisdom. Keep wisdom on the highest available amulet level you can get (or if you don't like waiting above it with an extra point), but focus on maxing out mental power. After that I recommend endurance as it is good for everyone and you will be using your ring to attack anyway and a few points in Agility or Strength doesn't get you anywhere. Accuracy is pointless for this build in my opinion.

Disclaimer: It is possible to make hybrid builds work, but specializing makes things easier on higher difficulties.

I think this is pretty much tied in with the "classes" at the attributes. Good news is though you will have more points in the game than you need in essence.

Technican route:
Max firearms, Max traps, Max Phial throw, Max Bomb (I skipped Anointed weapons as I don't like the mechanics, but it is okayish if you max One handed blades and have the agility to back it up). I went for maxing Roll and then One Handed Blades, Long Blades and Shadow Burst.

Gameplay wise I think all this is an overkill as 99% of the time you will only use Shoot your gun, Bomb at max fury and Roll. If I wanted to save ammo or just felt like the enemy is trash then use one handed or long blade and toss a bomb when fury is high enough. Or save up the fury and use toss first at a group.

Mage:
Divine magic ring, Lightning Dash, Shadow Burst, Storm, Stasis, Shield of the Enlighted, Magic Healing. Then pick a weapon, game recommends blunt ones for the full priest/mage experience if you ever run out of mana (you won't). The only issue with this build is that your companions won't care about your stasis at all which completely ruins normal Stasis and will make your "ultimate" fury ability Storm underwhelming compared to the two other abilities. Sadly since everything is linked to Attributes and you need to max out two of them and would need a third for a proper weapon AND you will mostly use a divine ring anyway there is not much wiggle room here.


Warrior:
I recommend one handed blades, long blades, fury, roll, firearms, anointed weapons, bomb (and picking accuracy as your third).
The priesty type would be one handed heavy weapon, two handed heavy weapon, fury, magical healing, divine magic ring (filler), lighting dash, shadow burst (and picking wisdom as your third)