Stats explained

It depends entirely on your specific class and the goals you have. There's no generic answer.

Strength is generally best for humans, dwarves and half-elves, but if you're playing a human infiltrator, the only way to get really reliable stealth is to prioritize agility first (unless you got an obscenely lucky statroll). If that's important to you, you could prioritize agility instead of strength.

For elves, strength isn't a priority because unless you can get 16, you don't get a damage bonus. Exceptional for elves is 15-16, so you might get exceptional (15) and have nothing. You only need 'good' to have no damage penalty, and depending on your class, it could be perfectly feasible to roll good strength at 3rd priority. But if you're not comfortable diving head-first down the well in case of an unlucky roll, the safe bet is to go strength 2nd. But then you're probably gonna be working with like 85-90 hp/stun, which is fucking awful.

Half-giants should never prioritize strength. I'm tempted to say that it should be your 4th priority. Exceptional HG strength is 48-50, and below average is 36-37. The difference between 50 and 37 strength is something like +15 vs. +17 damage. Strength scaling hits brutal diminishing returns around the mul level. Muls can safely prioritize it 2nd or even 3rd, although 4th might be a bad idea as you could end up with the strength of a strong human, and then what's the point of playing a mul in the first place?

I don't know why you would ever prioritize strength on a merchant. You should never be doing any combat on a merchant, so who cares if you're always heavily encumbered? Prioritize endurance first to hedge against backstabs and shit like that, then wisdom second. You can carry quite a bit even with poor strength when it's irrelevant what combat penalties it gives you. The merchant classes are strictly and exclusively non-combat, and it doesn't matter at all if you have huge penalties from encumberance and spastic agility when you're worthless in a fight anyway.

I wouldn't generally prioritize wisdom 2nd on spellcasters unless it's Krathi Destruction or something like that. On a character that does its killing with damage spells, it's undeniably nice to have 100 mana so you can cast two mon spells. For anything else, it doesn't really matter whether you have 85 or 105 mana. You're not really casting mon stoneskin anyway, right? For most spells, the power level only really determines the duration, and you usually don't even want them to last as long as possible, you'll be dispelling them off yourself anyway before they run out, or you can recast them when they wear off if you're still doing shit that requires them. Getting your physical stats high enough to be meaningful is much more important than squeezing out a little more mana.

Wisdom is a junk stat for anything other than maximizing scan, or maybe playing a totally non-combat psionicist. Back when we had full magickers, it was worth prioritizing wisdom because with no combat skills, it's kind of irrelevant what your strength/agility is unless you were trying to be a brawling Rukkian or a cheesy Krathi with fifteen enchanted items utilizing extreme stat levels to simulate combat skills. Now that everyone's a raider or scout or whatever on top of their magick subclass, you're just gonna get more out of prioritizing your stats the exact same way you would with a mundane character of the same class. In most cases, you don't use your spells at mon level unless they're damage spells or invisibility, or fringe stuff like the poison spell which you're not likely to be casting twice in a row anyway. For most buffs and utility spells, you just cast it at half power for 20ish mana and it really didn't matter what your maximum mana was because you're not likely to be doing shit that requires spending a full mana pool in one go. That was something that mattered with full-guild mages and sorcerers.

For psionicists specifically, their skill timers are so insanely long and their focus pool depends on wisdom, so if you're not playing something like a raider/psionicist, you can consider prioritizing wisdom. Like if you're some aide with a psi sub, you won't care about your combat stats. Psionicists don't generally synergize well with mundane combat anyway, it's usually something you pick for a plotting/political character where your physical stats are irrelevant. Since psionic skill timers are three times as long as regular ones, and since they don't have the plateauing issue that combat skills do, it does actually help a lot to have high wisdom. You may also have to maintain multiple different psionic skills at once, and if you have below average wisdom, that can lower your focus cap so much that it becomes problematic.