Everybody says eneloop, so that's what I bought, plus a smart charger. I've nothing to compare it to, but no complaints so far. I decided to get the normals rather than 'pro' which are for high-consumption devices. I didn't really think the Steam controller counted as such, and the pro version has a shorter life expectancy overall.
Does anyone else find they avoid using the grips for anything? I hold the controller by squeezing its very bottom edges into the heels of my palms, giving my thumbs the top-down access to the trackpads that Krejlooc advocated for early on. This leaves my last two fingers free, but too far away from the grips to use them. They reach that far, but can't exert a clicking pressure once they get there, without adjusting my grip every time. As a consequence I always configure against them. If I need to change my grip, I'd love some advice on how!