On my 10-Frame hive last year and now since today, I've used a BM plastic hive top feeder. It holds a gallon easily, and maybe even two gallons (but I've not tried that much in it).
For the 8-frame hive, I got a Mann Lake hive top feeder. It is made differently, with the opening in the middle and a gallon-sized holder on each size. It is screened in so that the bees can't get out and they have something to climb on so that they don't drown. I will probably get one for the 10-frame, as the plastic one seems to want to warp, and it didn't want to seal good this year.