I like giant vallsneria to get some height, particularly in a tall tank... up the side or back and over the top if you let it go.... if you are keeping the water on the low end of the ram range at least.
Crypts (lots of varieties to choose from) and amazon swords (they get pretty large not just tall so one, maybe two max for a 20 gallon... I found two too much for a 37 tall).
Aponogetons are nice and a little less large than swords... I would choose them over swords if I were doing it again.
Brazilian Pennywort can be planted as a stem or floated, good versatile plant, I tried both and choose to float it now.
Dwarf lily (bulb) is on of my current favorites but I haven't' had it long enough to see how it will do. Green with red spots on the leaves, interesting variety.
I just go and look at the plants and pick ones that I like the look of and try them out... I've have about 30 varieties in my tank at one point or another in the last 5 months. Most work, a few I didn't care for so I am down to 20ish. Those listed above are my favorites... I've gotten away from stem plants for a variety of reasons, preference for leaves mainly.