A little background info: I'm just about to graduate university with a software engineering degree. I have a fair amount of experience with more "serious" programming languages (C++/Java/C#), plenty of theoretical background, and good learning ability. I have minimal experience in web development as a whole (some JSP/JavaScript), and only very little with game engines (used Slick2D once to make a simple platformer, and had a brief introduction to 2D XNA).
I was recently approached by some people who have a game idea, and are looking for implementors. The game is a 2D turn-based strategy web game with a persistent world (meaning a DB backend would likely be required to store world state, user info, etc), and several UI interfaces.
I know this is a rather vague description, but based on this, which free game engine/framework/toolkit should I be looking at? Or at least, which technology (Flash/HTML5/JavaScript) would make it the easiest for me to develop this game?