Definitely Haku.
The kid doesn't have the best rep due to losing to Wendy, but people don't really realize just how much the loli had going for her.
First, she can completely negate his insta-win hax that even Natsu was completely helpless against.
Second, her built-in Irene could overwrite his control over his plush army which completely overwhelmed Wendy.
Third, and the biggest one, his crush on Wendy prevented him from fighting her seriously. Sure he said that he wasn't gonna hold back or whatever, but that statement can be thrown out the window after he deliberately took her roar attack because he wanted to feel her breath. Even their fist-clash, which is often used as proof that Wendy can react to him, was most likely him fooling around as instead of being shocked that his prized speed was reacted to, he just gushed about how he and Wendy touched each other's hands , indicating that he purposely let her clash with him (also something to note, him getting a "Rawr" out could also be an indication that he slowed down before the clash). For even more evidence that she's not relative to him, he followed that clash up by easily evading her wing attack + she needed to trap him in a wind cage to land sky drill (and this was on top of him already being pinned down by his own plushies).
The reality of the matter is that Haku is objectively the fastest character in the verse (based on feats/statements), and combined with his plush magic, the most lethal mage in that labyrinth. The Dragon Gods can probably have their speed scaled to his since they're the verse's God tiers, but there is no reason for his peers or anyone else to scale to what is one of his magic's specialties just because their other stats are better than his.
Edit: so I went back and read the chapter again. He actually makes that comment that he's gonna get a little serious after walking into her roar, but "a little serious" (his words) doesn't really change much, especially when he's slurping over the fact that he touched Wendy's hand mid-battle + the fact that he opted to pin her down with his plushies to avoid hurting her.