I know there are more politer forms like itadaku but just focusing on the masu and masen aspects, which is politer? What are the nuances? Which is more forceful-sounding?
As so often, it depends on the context.
Just translating them into English should be enough of a clue. 「～くれますか」 is simply "Will you give me ～?" (or "Will you do ～ for me?" in the case of 「～てくれますか」). 「～くれませんか」 is simply "Will you not give/do for me ～?", which of course can be rearranged to be "Won't you give/do for me ～?"
I would say that more often than not, ～くれませんか is a little softer because it is more like you're politely asking for help with something, and ～くれますか is more straightforward and blunt. But again, depending on context, different meanings can come across. If your friend asks you
￥1000貸してくれない？ → Won't you lend me ￥1000? (= "Can I borrow ￥1000?")
they are asking a favour of you. But if your boss asks you
17:00までに書類を揃えといてくれない？ → Won't you prepare the documents for me by 17:00?
s/he's obviously not asking you to do them a favour; it's a very polite and indirect command.
Now if you substituted くれる into both these sentences, they'd have a completely different feel. "Are you going to do ～ for me?" This sounds less amicable, like the asker just wants to know the facts. It could even convey a bit of annoyance of unmet expectations. ("Are you going to lend me the money (or not)?", "Are you going to prepare the documents (you lazy worker)?")
Lastly, just to note, the polite (respectful) form of くれる is not いただく; it is くださる. いただく is the polite (humble) form of もらう. Refer to this post for more information on those.