If you ever seen Deathbattle you should know already what it is all about. A lot are extremely close and others just one sided. There's some we agree on and there's those we don't. Which Deathbattle do you disagree on and why.
People need to realize that the animated battles themselves are meant to give you something exciting to watch, not to accurately gauge how the match would go according to their opinions. This is directly addressed with Superman vs. Goku; despite a quite close battle in the animation, it is pointed out explicitly that the difference between their strength, speed, and durability is "not even close".
Anyway, Wonder Woman should have beaten Rogue. Even with Rogue equalizing the physical difference, Diana is still a far superior fighter, and it has been pointed out that it is her incredible skill that makes her quicker in a tussle than even Superman. Not her natural superhuman speed. While the "Faster than Superman" thing has been contested, the reasoning behind her skill increasing her effective speed is sound enough.
Felicia would have probably run a train on Taokaka. This is simply a result of her verse being considerably more powerful than Tao's.
Super Shadow would kill Vegeta. Faster, stronger, more powerful.
Bison would beat Kahn. They didn't address the frankly insurmountable physical advantage Bison has.
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I agree on Wonder Woman beating Rogue, probably agree on Bison beating Kahn too (Kahn seemed too overexaggerated/wanked in that fight).
Other fights like Thor vs Raiden and Samus vs Boba Fett were just plain spite no matter how you put it (could've at least given Raiden his staff weapon or hammer from MK4, not that it would've made much of a difference). Also, yeah. Shang lasting as long as he did against Akuma was lolzy, but it was done to keep the battle entertaining and exciting; the same reason they made Goku vs Superman as evenly matched as they did in the animation.
In the Doomguy vs Master Chief fight, they seemed to ignore the Doomguy's gravity gun, which would've been able to catch Chief's plasma grenade in midair and throw it back at him at the end of the fight. The mystical weapons in both versions of Doom 3 (artifact, soul cube) were also mysteriously excluded. Doomguy could've just slowed down time with the artifact and fired a BFG round before Chief could ever use a bubble/drop shield.
Yoshi vs Riptor was silly with the way it quickly ended, not that I'm denying that Yoshi would win. Ironically, I actually made a versus thread on VGV at one point of the two dinosaurs before there was ever a death battle of them.
The logic behind that, IIRC, was that the Blade of Olympus, while of divine origin, was forged of earthly being. The implication is that it would require something from another realm of existence, such as Heaven and Hell, to actually do away with Spawn since he is essentially a product of Hell.
I don't quite know enough about Spawn to comment further, so I have no real opinion on it. As for the "no gods exist outside of Olympus" shtick, this only applies to the God of War continuity. When combining realms of fiction, you have to account for both as thoroughly as possible. In this case, Olympus, Heaven, and Hell were all applicable.