Great topic. My favorite for the past 13 years since I started by first tank were Stargrass, E. major, green Cabomba, Bacopa caroliniana, A. boivinianus, Anubias barteri var. nana and R. macrandra. I just find these striking in appearance in any aquascape.
Since restarting my aquarium hobby two years ago, I've come to really love the look of Glossostigma, Riccia, R. wallachii, Ammania gracilis, H. difformis, all Ludwigia spp., Hottonia palustris and Eichhornia diversifolia to name a few. At one point I had nearly 30 species of plants in my 90 gallon tank! The big plants won.
I eventually trimmed down the assortment to just 10 species of plants. I liked the look, but I really like so many different plants. So, now I have a second (25gal) tank at work, and I'm planning a 30" tall, 225 gallon tank at home to house my favorite, must-have, large Echinodorus, stargrass, red cabomba and Discus, about eight of them with a hundred cardinal tetras...but mainly for the plants.
(Somebody make it stop....!)
...I'm all better, now.