The non negotiables for me are the lifestyle habits.
Drink water (or herbal tea), prioritise getting a good night's sleep and eat more fruit, veg, nuts and olive oil. It is noticeable to me how much better I look and feel when I have those three things under control. A few nights of bad sleep or eating crap and I look significantly older.
In terms of products or habits I'd say:
Cleansing my face properly every day and using a flannel to do it for gentle exfoliation.
Vitamin C serum in the mornings. I've noticed a big difference in the brightness of my skin since adding this.
Moisturising with something in the evenings.
I think consistency with a simple routine day in and day out for years is more important than which products you use. I know people that have used Nivea Soft for years as their moisturiser and have amazing skin so it obviously suits them. It would make me look like an oil slick so find what suits you.
I do wear SPF 50 every day, including on my neck, decolletage and back of my hands but I haven't noticed any difference day to day..... I do it to hopefully notice the difference in 20 years!
On another note, I have been taking collagen supplements of various types for 18 months and have noticed absolutely no difference so will be stopping them once my current supply is used up. I'll be sticking with my multivitamin and omega 3 supplement but that's it.