From my experience here in the UK;
In the fully hardy group it is the small flowered shrubby species and cultivars. The real star performers are Nachtvlinder, Pink Blush and Blue Note.
Among the hardy except in a bad winter group then it has to be S.patens or Black and Blue.
In the tender group it is easily S.discolor which if kept a degree or two above freezing will bloom non-stop. Honorable mentions go to Amistad, Love and Wishes and S.oxyphora.
