Eijun and Miyuki, not just for reasons of canon but because of what sakuhin actually means, and their growth over the course of the series. Also that Eijun has transmitted ideas about battery to Okumura that he got from Miyuki but not from Chris.
Not that I have anything against either Chris or even Okumura, but it's just a matter of what the manga says about their battery from day one. Key aspects of this are also not technical but personal. Eijun wants to form a battery with Miyuki, he goes to Seidou to do so, and now in current manga we've been told that this basically what a battery is, explicitly. Eijun's absolutely driven to make THIS summer happen as well.
There are a lot of points Miyuki recognises Eijun's ability long before they go to Koushien, though. This is really important when trying to claim Chris taught Eijun everything and is responsible for all of Eijun's growth (how did someone put it? Miyuki reaps what Chris sows?) This is a complete fallacy. If you go back to something like chapter 20, 21 of Act I, Chris is really abusive to Eijun. He tells him he will never be the ace, twice, and to go change schools as he can never compete with Furuya. In Chapter 22, Miyuki talks to him and points out that Eijun is the kind of pitcher that a catcher should most want to work with, or words to that effect. Not only is this clear evidence of Miyuki recognising Eijun way before the Komadai game in Act II, it's also Miyuki recognising Eijun before Chris does, and prompting Chris to take action. We see his clear eyes first in that scene. Miyuki is the catalyst.
After Eijun has yips, too, Miyuki is the catalyst to get Chris involved. Chris tells Kanemaru that he doesn't want to make too big a deal out of it and something like he'll get over it because he's that kind of guy. It's only after Miyuki asks him to intervene that he starts teaching Eijun the outside pitch. Again, Miyuki is the catalyst. Without Miyuki, it's probable Chris wouldn't have done anything in either scenario. Not an ideal battery.
It's mysterious how often people pretend these things didn't happen, when it's so easy to read back and see that they did.
Moreover, Chris doesn't come into the second string game really to form a battery with Eijun so much as to save his ass. It's more a teacher showing the student, and showing Chris's talent for fixing the situation rather than Eijun really shining as his partner. A battery is a partnership - it was Miyuki, not Chris, who termed it that way first, though. That game is more about Chris's swan song in a way than Eijun, showing us what kind of a catcher he was/could have been, had he been fit. I never got the feel there was an actual 'battery' connection there. Teacher and student, yes, but that's not the same thing at all. Maybe it's because Chris had to be forced into dealing with Eijun initially.
Basically I support Eijun and Miyuki's battery because from my perspective, Miyuki's the catcher who never told Eijun he couldn't be ace, unlike Chris, and who has never tried to change Eijun's personality (unlike Okumura). Miyuki's accepted Eijun as he is, and hasn't capped his potential. Just because he doesn't squeal and hug him every time he gets a strike out doesn't make him unsupportive or disconnected. Miyuki's also the person who cares more about getting the pitchers to Koushien than his own health (Seikou game, tenth inning). In a sense he's far more engaged with the pitchers overall.
It's also Miyuki's lessons (Sakuhin and aibou/partner) that Eijun tries to pass on to Okumura, not Chris's. We never see Eijun mention Chris in front of either of the first years, yet we see him actively excited talking to Yui that he gets to practice with Miyuki in the bullpen.
As for the idea Chris taught Miyuki everything, the numbers are entirely Eijun and Miyuki's work, nothing to do with Chris at all (aside the 4seam). Ochiai taught the change up, but Miyuki worked through the winter on the numbers with Eijun. Miyuki also brought Chris to the game to see Eijun pitch against Ooya, because he wanted to show something Chris hadn't seen - evidencing Chris is completely detached from what Eijun is learning or doing except when someone else (Miyuki) brings him into the equation.
On the subject of other batteries, I like Ono and Kawakami as a battery, but that's partly because Nori is the most interesting Seidou pitcher to me, and Ono works really hard in Miyuki's shadow. Somehow both of them operate that relief position but they seem stable when they work together and I like that.
Miyuki and Furuya work well but not as well as Miyuki and Eijun, because Furuya like Miyuki tends to hide things like injuries and such. There have been a few mishaps there which make me think it's a less strong battery. If Ono could manage Furuya's pitches, he'd probably be a better partner for Furuya. Maybe Yui will be, I guess we'll see.
I'd love to see Toujou pitch more but anyway.
I personally think Furuya and Okumura work better as a battery in what we've seen thus far than Eijun and Okumura at the moment. It's funny that there's nothing on Koushuu's page for his interaction with Furuya. But I'm not sold on the Yui is already out of the race theory.