As far as technology goes, Star Trek wins outright. However, the show was always focused on exploration and diplomacy, so in comparison the federation military is pathetic in almost every way compared to the others. In an all out war, the republic would obliterate the federation based on the number of planets held, the higher technological diversity because of the number of member species and the higher military potential. Also, since you're going clone wars days, it's a no brainer. Star Trek has them beat on tech, but the military forces are nothing compared to the forces we've seen in Star Wars.
Those are the two juggernauts... next we have to consider Mass Effect. I immediately dropped them from the big battle above because of their reliance on the Mass Relays to slingshot across the galaxy. Star Trek and Star Wars use warp travel and hyperspace routes respectively, which allows them to jump massive distances from almost any location. The deployabiltiy of troops with this technology is ridiculously easy. In the Mass Effect galaxy, the big points to hold would be the relays themselves, if those were lost, the war would be lost. As far as tech goes, they are definitely the weakest of the three mentioned so far, and as far as I know the scope of the galaxy is pretty small in comparison to the two as well. The size of the combined citadel fleets might match the Federation in size, but they wouldn't last in a war with the Klingons, much less the entire federation. The tech is just too far ahead. It would be like fighting against modern weapons with tech from the first world war.
Halo I don't think is in this fight at all. The scope of the galaxy here is way too small to even compare against the others in my opinion. As far as technology and organization goes, they could definitely give the Citadel fleets a run for their money, but in the end the scope of the milky way galaxy in Mass Effect is way bigger than the scope of the galaxy in Halo. Not to mention that shielding and mass accelerating weapons are common armaments in all militaries that have discovered the mass relays, while those are specially reserved tech for the Spartans in the Halo universe. I admit I don't know much about the size and technology of the covenant fleets, so I can't say much about how they would fare. On the ground they would be slaughtered though.
So in an all out war I'd stack it like this:
Star Wars > Star Trek > Mass Effect > Halo
In terms of tech and smaller scale battles like this:
Star Trek > Star Wars > Mass Effect > Halo
Though most of my assumptions on Halo are based solely on armaments of the ground troops. Someone with more knowledge of the lore will have to school me on the specifics of how a space engagement would go.
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