I'll go with Raven. Even though I like both characters, the truth is Raven has CONSIDERABLY more combat experience (the very vast majority of Twilight's [mis]adventures are comprised of Slice of Life shenanigans) and, let's be honest, Twilight has NEVER beaten a major villain or monster on her own (though, to be fair, neither has anybody else in Equestria as that would be counterproductive to a show that runs on Friendship Aesops). Regrettably, Twilight is entirely too dependent on her friends when it comes to combat and Death Battle rules would strictly prohibit her having her friends at her back here.
Raven has beaten Trigon himself on her own at least once. And the only villain in Equestria even coming close to Trigon's power would be Tirek when he had the power of MOST of the rest of Equestria. Twilight needed to have a serious power boost from the other three princesses just to keep him from Curb Stomping her and, even with THAT, the BEST she could do was fight him to a draw.
Plus, Twilight, with VERY rare exceptions, prefers to save violence as an absolute last resort. Raven, on the other hand, while certainly not evil, is much quicker on the trigger (so to speak) than Twilight typically is.
Ergo, this fight would be a complete Curb Stomp in Raven's favor.