I'm not sure on which plant is my favourite, but I do know that I have way too many in my mayor's house XD
I really like the morning glory because of the flowers being my favourite colour. I also like another one... I can't remember what it's called, but in game it looks like a small round white pot with red flowers in it. I think that one looks nice, too.
I love plants too. I think they really soften the look of a room. I usually try and squeeze in a pothos somewhere. That plant was supposed to add to your luck in WW so I am trained to do that. I don't know if that is still the case in NL.