Is the question whether Marvel needs one of these properties to help build their Cinematic Universe? Or is it whether these properties are in need of what Marvel Studios brings to the table in order to produce a quality film?
Either way, "need" is a strong term. Marvel Studios certainly doesn't "need" any of those characters. Clearly, they are doing just fine with what they have, and they have barely begun to tap into the stable of characters at their disposal.
Do the X-Men "need" Marvel Studios? After three generally well-received X-films in a row, Fox seems to have the franchise trending in the right direction.
Does Spider Man "need" Marvel Studios? ASM 2 had its moments, but most seemed to be disappointed. That follows a fairly well-received Amazing Spider Man, which was a bounce back from the putrid Spider Man 3. Yet, Sony produced what many consider to be one of the best comic book movies ever with Spider Man 2, so they are certainly capable of producing a quality Spidey film.
Which brings us to Fantastic Four. It's kind of hard to say, since we haven't seen the new film yet. But, we know the history of the Fantastic Four on film, and we all have our opinions of what direction Fox seems to be taking the property. Of the three properties, it would appear that the FF "needs" Marvel Studios the most, just judging by history. But, the reboot could surprise us and surpass all of our expectations.
Ignoring "need" and going strictly with "want," I would like to have Spider Man back home. He's Marvel's signature character, and I think Marvel Studio's knack for blending action and humor would suit the Wall-Crawler perfectly.
The fan in me would also like to see the Fantastic Four at Marvel Studios. Not only do I want to see an MCU version of Dr. Doom, but I'd also love to see a scene with the Science Bros, Reed and Pym talking way above everyone's head. (Throw in Selvig too)
The X-Men are fine where they are at Fox. There are so many mutants and characters directly related to the X-Men: the X-Men themselves, X-Factor, X-Force, the New Mutants, the Morlocks, the Acolytes, the Brotherhood, Excaibur, (Alpha Flight?), etc. They are a cinematic universe unto themselves. While I would like to see Storm (not Halle Berry's version) in a Black Panther film, the character is better served where she is.