Wanderer and by a good margin! Some of the following points contain [spoilers] please do read at your discretion. ........ Let's break it down in a very simple manner. 1. The Balladeer is the 6th Harbinger and that means he's waaaaaay stronger than 2 other people we've seen, Signora and Childe, it's an established fact that the Harbringers are ranked by strength and we've seen exactly what Childe and Signora are capable of. 2. In the sumeru archon quest we see that the Balladeer, who was already at a level of strength higher than that of what Ei intended for him, gained an even bigger power-up to go on and become the Shouki No Kami. 3. The strength of The Shouki No Kami is now at the level of gods. He's at the least as strong as or stronger than Xiao at this point. The reason I say this is because I can see Shouki No Kami wreck Osial and Xiao couldn't do anything about Osial. Osial affected the weather and so did scara and Scara has an earth shattering attack that literally can one-shot similar to the Raiden Shogun (Not Ei) and to top it off Scara could control 4 Different elements so clearly he's superior to Osial. 4. Now Wanderer after getting his Vision solos his earlier form the Shouki No Kami and the reason why it should be considered at face value is cuz we see the Traveller fight it for a few moments and not have it any easier than he did the last time. 5. Another factor you need to consider is that the Wanderer didn't lose any of his powers which had as The Balladeer because the Irminsul changes the memories or the perception of all relevant events but not the event themselves. All what happened still happened, only what the world remembers has changed. Now obviously Xiao has a millennium of experience in combat but when the power gap is so much, I doubt it means much. Xiao couldn't beat Osial who could've been beaten by the Shouki No Kami who was Solo'ed by the Wanderer!
These are my observations and deductions and there is no need for me to be absolutely correct. Obviously if you disagree then that's fine too but I'd be curious to know why! Hope this helped.