The Costco 6V used to be my go-to but the quality of those batteries has gone to crap. I recently had to replace the batteries in my 36V golf cart and after some research, I ended up with 6 of the Super Start GC2 - EV125 FLA 6V from O'Reillys. They're made by East Penn and are built for use in man lifts and such so they have thicker plates and a tad more oomph but are the same dimensions as any GC2 battery. My shop has an account with O'Reillys so it helped offset the cost. So far they've been working great but obviously time will tell. I've thought about switching to 6V in my pusher for the house when the time comes but the trio of group 31 Duracell AGMs in it have worked great with no leakage so I'll probably just replace with the same.
One of the easiest ways to check the build quality of a standard 6V battery is by weight. The Costco (Interstate) 6V is one of the lightest ones out there, for a FLA battery anyway.