I'm not too familiar with research gate, but for the rest:

Quora: By far the worst when it comes to objective facts. While there are gems here and there, most upvoted answers read like pep talks, and usually don't have much content beyond that of a decent pep talk. Go here if you want a pep talk or a popularization of some scientific concept.

Stackexchange: is where you want to be most of the time. The answers are good, and their accuracy is much better than anywhere else on the net. If you're asking about a topic anywhere close to mainstream in academia (or if you're interested in computers), you'll find an expert on that very topic.

Lesswrong: Honestly, lesswrong is too small. Unless you're going to ask about niche topics (EA, Singularity, Cryonics, and the like), there's a good chance nobody here is an expert on what you're interested in. If you are interested in the niche topics, you'll find people willing to give you detailed answers. But beware of only getting one side of the story. The answers you'll get don't necessarily reflect expert consensus, or even rationalist consensus (See yvain's opinion polls and how they match up with the comments you read. Also, check out all the other rationalist hubs. For instance, the tumblr rationalists have a markedly different atmosphere.).

Discussions are are different medium than the straight "question and answer"-format. LW leads itself to discussions.

StackExchange has the advantage of having quality standards for questions. I think it's often the best place if you clearly know what you want to know.

I use Quora only when questions are off-topic on stackexchange.

I'm not familiar with Research Gate Q&A.

Stackexchange: Highly preferred among the four choices listed. Works especially well where there is a clearly constrained question that could actually be answered in a constructive way. Several of the research questions in the original post might get constructive answers if posed there.

Lesswrong: Good for open-ended discussion, of a highly critical nature - community standards expect a certain level of rigour.

Quora: I've never used it so I don't have any comments there.

Researchgate: I have a research profile with my CV on it, and all the publications I am allowed to distribute. But I find that the Q&A section in my field is mainly something like "I need a specific kind of resource, can anyone help me find it?", and often answered in uninformative or useless ways (e.g. "here is a vaguely related paper, go read it and see"), by people who just seem to like answering questions (cf Yahoo answers and all of its ilk). Very handy for keeping up with new publications (not paywalled) from people whose research I follow.