"These are the best potatoes."
Translation:Ăștia sunt cei mai buni cartofi.
For masculine plural, aceștia (formal) can replace ăștia (informal) and, for feminine plural, acestea (formal) can replace astea (informal). They are the equivalent of saying these in English like in this sentence (unattached to something).
If you would instead want to say These potatoes are the best potatoes (notice that these is now attached to potatoes), then you can say acești cartofi sunt cei mai buni cartofi, and aceste is the female variant of acești. Something like aceste flori sunt frumoase (these flowers are beautiful).
I hope it answers your question of why it isn't correct and that it clarifies their usage for you.