Hands down... Zinnias! I used to hate those things. You could not pay me to grow one. When I moved here I bought about 5 dreamland zins. We were new to this area and the one nursery we had sold them. It was really about all that was available at the time. They turned out so beautiful that I wondered how I went all these years without planting zinnias. Then I joined garden webs forum. A man by the name of ZenMan had a thread about zinnias. I went through that thread and the absolutely amazing things he breeds is just awe inspiring for me. He has inspired me enough to collect zinnia seeds. Don't be fooled, I don't just collect, I grow. I had over 15 different zins in my gardens this year alone. I really never knew how beautiful, surprising and easy they were to grow. One of these days I will definitely do my own breeding. I'm still waiting on my greenhouse, lol. And a huge thank you to Zenman for opening my world and showing me the way.
Other than that, my next truly fave things to grow are cleome, snapdragons, pentas (this year cultivated my love of that plant), petunias (must be frilly grandifloras), celosias, gerbera daisies, gazanias and roses.