For something different, how about a pair of brevis shelldwellers ? I don' t think you'd be able to put anything else in there with them though. They are a pretty little cichlid.
Or 3 or 4 or those blue eyed rainbows ! I believe Roan said they only get about an inch long...and they're a VERY flashy fish from what I've read about them. I bet they'd look nice in a well planted 5 with a small piece of driftwood.
Of the 3 that you've listed...I think I'd go with the blue eyes.
I saw endlers at an lfs recently and was SOOO tempted to get a small group...once I can find someplace else for the 3 female guppies in my 20long ram tank.....maybe I'll put a nice school of them in there.....
My vote is for the endler's, I just bought a trio and they are so cute. Mine are orange and black (takes me back 20 years to my high school colors). Hoping to have some babies, one of the females looks to my untrained eyes to be pregnant.
I have one betta and about 3 apple snails in my 5 gallon and he's so happy. Originally, I had dwarf puffers in there (3) and they were fine as long as each had their own "landmark" (one figurine, two plants). Cleaning was kinda difficult though because they were so curious of the vacuum, but those were definitely my favorite inhabitants. So I guess my recommendation might be just the betta- to marvel at the sheer size of his bubblenest- or some dwarf puffers (I just love those little guys).