Of course Skinceuticals will always be the gold standard, so if you can afford it: fill your boots! If you can't, I do think there are some decent alternatives out there. Whilst this time last year, the Paula's Choice would be my top pick, I think ultimately it's too expensive per ml to offer a truly affordable take on ascorbic acid. It's probably no surprise that my top recommendation is Geek & Gorgeous because not only did it have the most impressive stability out of the products I tried but it's the most affordable. On this occasion the cheapest was actually the best for me, so go forth and try that product!
Have you tried any of these formulas? How did you get on with them?