I asked a partial question on stackoverflow, and then realized that this would be the place to get a better answer because this is the game dev website, and you all may know more than the programmers over at stackoverflow about this subject.
Efficient as in up to date: what is used today by most people, what should be used by a group of people who aren't that experienced in programming (AKA easy, well documented) this isn't really a yes or no question, more of an opinionated answer. Some people hate flash but others love it, I'm unsure of what I should use for a Browser MMO RTS and would like your opinions on it.
Just to clear this up to people who don't know: an MMO RTS usually means its a game with not too much movement, but nifty animations when you scroll over pictures. Also has pretty nice art (dealing with pixel art or whatever will be an issue...).