Hmm, that's a tough question. Two beloved comic characters were given horrible short shrifts to cram into a sequel. Both also with interstellar origins (and therein lies the problem!).
Phoenix made sense within the movies in that it was just her evil inner self. And by making her evil with no exterior reason, quite frankly the less time dwelt on it the better!
Venom however had his (unexplained) alien origin, his (abbreviated) character origin, but got zero screen time and didn't appear until the last 20 minutes. I'd go with Venom.