There are SO many Vitamin C serums out there. Someone is probably going to suggest the SkinCeuticals C E Ferulic at some point, but (1) not all of us have that kind of budget, and (2) in my opinion, there are perfectly good products for less (sometimes a lot less).
Personally, my favorites right now are Maelove’s The Glow Maker and Hyper Skin Hyper Clear. The Glow Maker is a SkinCeuticals dupe with a few cool extras, while Hyper Clear is specifically designed to address hyperpigmentation. So, Hyper Clear might be your best bet if that’s your main concern.
ETA: In case this is helpful and/or fun for people, I made a Vitamin C serum quiz not too long ago that will match you up with a (cruelty-free!) Vitamin C serum based on your particular preferences and concerns. There are 14 possible results! It took a while to make