maybe an ADF
or a Betta
I have kept a betta in a 1.5 up to a 5 gal tank before. a lot of people will argue thats its not ideal. but i've had bettas that didnt like large tanks. when i put them in the smaller tanks with a few plants and a hiding place, they were more colorful, ate better, were more active, and made bubble nests like mad. when i put them in the larger tanks, they would hide in a corner of the tank, usually behind the heater and not come out.
and then i've had other bettas who hated small spaces and thrived in a larger tank (10 gal and up) and even liked being in a community tank.