Epson Picturemate 225 is what I would probably buy if I were getting one today. I had an earlier version a few years ago that printed flawless 4x6 prints. Only problem with that one was, its pigment inks would clog the heads after a couple weeks on non-use, and a lot of ink would be wasted in head cleaning. This newer model uses dye inks and isn't subject to that problem. Nowadays I just use an Epson Artisan full size printer.
The Picturemate even has an optional battery
I also have (in a drawer) a Canon Selphy CP730. There are newer models now in the Selphy line, but basically they are a dye-sublimation printer. The special paper makes several passes through the machine, one pass for each color. The print quality was pretty good but mine always had a slight orange cast to the prints. Also be aware that the paper has two little perforated strips, one on each end, that you remove after printing; the end result is that the print is a little shorter than the standard 6" (about 1/8" is lost to each tear-off strip). It's just a quirk of their system and probably won't bother most people.