If we go by canon numbers, Mass Effect loses* simply because it tries to be semi-realistic (heat management, physics aside from Eezo). I love Mass Effect as a game and universe, but in a sci-fi tech battle they're toast. Every other faction has...
That said, Mass Effect has a few unique items...
*Excepting the possibility of Reaper indoctrination or other forces gaining access to technologies lacking in that universe
This leaves (again my personal opinions) the Ultimate Showdown of Ultimate Destiny...IN SPACE (or not) for the rest.
Star Wars dominates space. Period. Gigaton, multi-gigaton, etc. weapons simply tear through anything, even Borg. Sure, cubes laugh off megaton-scale weaponry (torpedoes, which I assume are more powerful than phasers/disruptors) of the Federation, Klingons, etc., but I don't think a "maximum efficiency" group would overbuild to resist weapons orders of magnitude greater, at least not initially. Speaking of the Federation, they build explorers, not warships--even though they're armed, they are not militarized.
Halo ships may compete with Star Wars if many weapons are in the gigaton range (e.g. Super MAC), though Halo on Wikia gives shipboard MACs as little better than Mass Effect (59 kilotons). Covenant and Forerunner weapons could even the score though--I have no idea on their yield. Given that Covenant ships "glass" (read: Star Wars Base Delta Zero) I suspect they would be highly competitive in the destruction department, but not in the defense department since a Super Mac (low gigaton yield--same source gives it ~50 gigatons) can "punch a hole clean through a Covenant Capital Ship" (and pass through that ship, destroy a second one, and cripple a third). Not sure what size of ship is being referenced, but "capital" implies something of significance.
Halo's assets include...
Star Trek's weaknesses have already been brought up. But like Mass Effect, their ability to steal technology and a few specialized items no one else possesses may give them an advantage, aside from those they already hold, which are...
Depending on where the fight takes place, factions could have specific advantages (or disadvantages). For example, without mass relays the Mass Effect faction is totally screwed. Strange stellar phenomena in the Star Trek galaxy they can deal with (after a show's worth of fuss of course) might make life difficult for everyone else. The various super-powered Force wielders in the Star Wars galaxy could also make life difficult for those unable to handle them, as could the Q Continuum--if they bother to interfere, that is. (Note: I consider the Q to be so powerful they're basically "author-weight" so it raises question of both fairness and "Really, seriously--this bit again?" from a storytelling perspective). Halo doesn't really talk about "negative space wedgies" much in the areas I have knowledge of (read: not much) but the Arrays exist. Mass Effect's pseudorealism also rules out any sort of stuff that's the "Problem of the Week" on Star Trek shows. If someone activates the Halo Arrays, well, everyone's gonna die.
Finally, outcomes depend on which version of each faction we use. Do we use the "latest in their canon timeline" (post-Halo-4, far-ahead Legacy Era, "reboot"/far more military Star Trek, post-Reaper-War Mass Effect)? Or do we use some other timeframe? Granted, picking and choosing timeframes makes it easy to change the power of each faction. So which is used matters.