Hold your ground, hold your ground! Sons of Gondor, of Rohan, my brothers! I see in your eyes the same fear that would take the heart of me. A day may come when the courage of men fails, when we forsake our friends and break all bonds of fellowship, but it is not this day. An hour of woes and shattered shields, when the age of men comes crashing down! But it is not this day! This day we fight! By all that you hold dear on this good Earth, I bid you stand, Men of the West!
I've seen a thread like this before, and i figure that i should display my opinion on who would win.
Many say that Halo would beat Mass Effect due to Halo's Covenant cruisers, great warriors, and of course Halo, but I disagree. Considering that Halo is including the Covenant (the bad guys) then Mass Effect should include the Reapers. And the Reapers would demolish everything.
Covenant cruisers?
A reaper is twice the size of one, and there are thousands of super-powerful reapers.
Warriors?
It wouldn't help against gigantic space stations that can annhilate everything.
And lastly, what about Halo?
You may think that Halo has the ability to destroy Reapers, but you'd be mistaken. Reapers are composed of half organic and half inorganic materials. Therefore, it's neither alive nor dead. Halo wipes out all life in the galaxy. Therefore, Halo wouldn't work against the Reapers, rendering it only effective on the living creatures in Mass Effect.
Bottom line: Mass Effect's Reapers > anything Halo has.