Reskilling. Being able to change what skills I have to suit different situations. Need more strength, bam. Need more intellegence, wow. Need to double jump, how about we keep that as a passive.

there are many amazing and fantastical answers, but truthfully the one that i would find the most interesting is what is your chance, what is your chance to hit a target, what is a chance to make a jump. What is the chance that i convince this person, that i pass this test etc. To truly know your own ability and likelihood of success would such great insight. Maybe a boring answer, but a interesting reality.

The ability to perform a dragon shout( minus the part where you have to absorb dragon souls for learning shouts). There are so many cool things you can do with dragon shouts like breathing fire, slowing down time, changing weather etc.

Inter-dimensional travel like the reality-jumping ability of Elizabeth in "Bioshock: Infinite" would be amazing. Truly a skill with limitless potential!

Giving buffs ! Yes! Helping people with healing and temporary power-ups with the power of my will ! Amazing.

The best one would be to be able to sleep as a secondary function. Like auto-charging, but for sleep so you are always refreshed and ready and never tired and sleep is happening in the background.

I believe the best video game skill would be Alchemy, because if the skill were real, then the "science" itself would also have to be real. Think of the possibilities, if alchemy were really a thing. You could heal injuries, treat conditions, improve the quality of life for people the world over...

...not to mention that whole "lead into gold" thing. ;)

I'd love to have regeneration skills! I mean, of course, we all can regenerate, but mostly it's happening too slow. I have quite a number of health problems and the ability to recover instantly (or just faster) would come in handy :)

For me luck would be the best. Without being todo good a most things in my life with a bit if look you can do big things... Or at least look like you are smart, clever, strong, handsome.... Well, not handsome... There isnt luck enough in the universe to change this ugly face...

respawn of course...lest something unexpected and bad happen to me

I'd like to be able to clone myself into many-like five or six at a time.
That way I'd be able to finally make a dent in my GOG game list! You know, finish more games than I buy...

Given that i'm a psychologist and also enjoy playing RPGs, and often in games where available I take the 'speech' route, and often play the different factions and allies/foes where to I take all sides and once I've gotten what I needed decide which of these deserves to be spared, maybe.
So I'd say a sort of speech and sneak oriented character, perhaps a shapeshifter. I'm not saying I practice this sort of manipulation on my patients, just saying I could.

I'd really like the ability to level up, in the sense that as I gain experience, I become stronger, smarter, more capable.

I'm pushing 50, and I can state with some measure of sadness that I am slowly de-leveling....

I know it's a little meta, but: Save Scumming.

Imagine being able to live your life in a way where you could try something, see the outcome, and if you didn't like it, just go back to a previous save and try a different thing until you were satisfied with the outcome.

It'd also essentially make you immortal, but not in a cursed way like vampirism or a boring way like actual godhood. You could also always just not revert to a older save state.

The only down side would be you'd have to remember to save often!