ONE beauty product you swear by?

[OneWorthyLemonCat]

I've quit my job and now have no income, which certainly focuses the mind on which products are worth it and which aren't.
I've downgraded my eye make up remover, I only use Simple face wash anyway, I've been using Medik8 eye cream but won't bother again when it runs out. There are various supplements and vitamins I think I can do without, as I get everything I need from my diet. Good shampoo and conditioner isn't worth it for me.
But the ONE product that is a non-negotiable for me, and I'll continue to buy it (using my savings) for as long as I'm not working, is Olay Max 24 Retinol night cream. It's had a marked impact on my skin, which now has a real glowing shine to it: https://www.superdrug.com/skin/night-creams/olay-retinol24-max-face-moisturiser-50ml/p/841556?gclsrc=aw.ds&gad_source=1&gad_campaignid=18152693967&gclid=EAIaIQobChMIubnm5oz8kgMVXJNQBh3AmSYLEAQYASABEgJW8PD_BwE
If you were in a similar boat, what ONE product would be a non negotiable for you?

Brown mascara. Looks at first sight in the mirror on passing like I haven't any on vs black, but still gives the appearance of thicker longer lashes. Has to be applied carefully to avoid clumping and in layers, but I think it's a great improvement on black. Haven't noticed too much difference between brands, I do like L'Oreal but if push came and I was totally skint I'd not be averse to eg Boots No17 or whatever.

My Clinique Dramatically Different Moisturising Lotion. I sometimes try different products out of interest when I need a new pot but ALWAYS come back to it.