I'm trying to get some starter and eevee shinies, so I usually throw my failed attempts to WT, the ones that it's usual to see me trading around are:
-Bulbasaur
-Charmander
-Squirtle
-Chikorita
-Cyndaquil
-Totodile
-Treecko
-Mudkip
-Turtwig
-Chimchar
-Piplup
-Snivy
-Tepig
-Oshawott
-Chespin
-Fennekin
-Froakie
-Eevee
I sometimes trade others that I find or I get back in trading and consider that actually are worth. I don't mind getting bad pokemons in exchange, I'm just doing it for fun and to fill my pokedex after all. Sometimes I even evolve the bad pokemons that I get and teach them some nice moves before sending them back to WT.
I even once sent a shiny Vivillon with sun pattern in it, it was just sitting in my PC box, and I felt a little guilty for that, and I thought that a shiny Vivillon maybe could make someone's day. I got a nice Japanese Ditto in return, so I think that it wasn't a bad trade after all.