If you are cycling a 55 with a fish-less cycle, kudos! Maybe it is time to consider killifish. And if you are not interested in breeding them, a bunch of less expensive male killies (at least at club auctions) are better behaved around other fish if there are no female killies to impress. Just make sure that companion medium-small tetras, livebearers or Corys aren't mouth-sized. :)
Killifish come from "fish of the kill." A kill is a Dutch - American name on the east coast for a small or medium sized stream. So killifish means fish of the stream, not some killer predator (unless you are a mosquito larvae).. ;)
With over 1,000 described killie species, local strains (collecting locations) and a few color sports what the most beautiful species is will vary among killie nuts. That is a little bit like asking who is/was the world's greatest soccer player, basketball player, left handed pitcher, home run hitter and so on.
If you don't wish to Google these fish listed below, scroll down to the URLs way below. More suggestions on keeping killies are in the sources.
Easier killifish, more or less plant spawners:
Aphyosemion ahli
A, bivittatum Funge
A. campomaanense
A. edeanum
A. gabonense
A. ogoense
A. primogenium
A. pyrophore
A. schoetzi
A. striatum
Aplocheilus lineatus
Epiplatys dageti
Ep. or pseudepiplatys annulatus
Procatopus aberrans
Fundulopanchax gardneri group Misaje, Innidere, P82, Makurdi
Fp gardneri gardneri Nsukka, Lafia
Fp. walkeri
Fp. spoorenbergi
Fp. mirabelis group
Scriptaphyosemion cauvetti
Script geryi
Script liberiense
Script schmitti
Easier annual killifish (who lay eggs in mud or peat moss that dries for a few months)
Austrolebias nigripinnis
Nematolebias whitei
Nothobranchius guentheri
N. foerchi
N. kortausae
N. eggersi
N. kilomberoensis
Simpsonichthys fulminantis
More challenging plant spawners:
A, australe gold or Chocolate (need less mineral in water to spawn)
A. batsei
A. cyanostictum
A. elberti
A. kekemense
A. jorgenscheeli
A. occelatum
A. wachtersi
Epiplatys ansorgei Masana
Lamprichthys tanganicanus
Rivulus agilae
Riv mahdiaensis
Riv rectocaudatus
Riv scalaris
Riv xiphidius
Riv sp. paryagi
semi-annuals: Callopanchax occidentalis
Fp. sjoestedti (blue gularis)
More challenging annuals:
Aphyolebias peruensis
Micromoema xiphophora
N. rachovii
N. furzeri
Gnatholebias zonatus
Renova oscari
Simpsonichthys boitonei
Simpsonichthys magnificus
Simpsonichthys picturatus
Simpsonichthys santanae
Simpsonichthys zonatus
Terranatos dolichopterus
http://theaquariumwiki.com/File:Aphyosemion_striatum3957.jpg
http://www.killiclubdefrance.org/squelettes/fichiers/Chauche_old/Aphyosemion%20bivittatum%20Funge.jpg
http://www.aka.org/aka/modules/newbb/viewtopic.php?start=20&topic_id=463&viewmode=compact&order=ASC&type=&mode=0
http://www.aquanet.de/AquaLex/Detail.aspx?guid=3c3d985e-b529-4082-82e1-9b93a9f2c1e7&t=1
http://www.aka.org/aka/modules/newbb/viewtopic.php?topic_id=733&forum=1&post_id=3428#forumpost3428
http://www.aka.org/wak/index.htm
http://www.alfanita.se/
http://www.itrainsfishes.net/content/
Hope some here catch your eye!
All the best!