Right then mate, for white fish, you want stuff that's fresh and bright. You're not trying to overpower it, are you?
I've been proper into home cooking lately, trying to get my head around all the different flavours. I used to just bung on salt and pepper, thinking it'd be alright, but then I tried adding some herbs to a cod fillet I was baking, and blimey, what a difference! It's like finding a new bit of code that makes everything run smoother, you know? It's really made me think more about what goes together.
For white fish like cod, haddock, or even something a bit fancier like sole, your best bet for herbs is usually pretty simple. Dill, parsley, and chives are always a good shout. They add a fresh, herby taste without being too strong. You can use them all together, or just one or two. Lemon zest or a squeeze of lemon juice goes amazing with them too, it really lifts the flavour.
Another one I've tried, if your looking for something a bit different, is tarragon. It's got a slight aniseed flavour that's quite unique and it pairs well with fish, especially if you're doing it with a cream or butter sauce. Thyme is good aswell, especially if you're roasting the fish with some veg. Just don't over do it with the thyme, you don't want it to taste like a Sunday roast!
So yeah, keep it simple, fresh herbs are your friend, and don't forget the lemon. Your fish will be spot on.