I've tried most of these and found Lucario to be the best partner for Blissey. Delcatty has the best synergy, but Skitty starts are about the worst thing in Pokemon right now. I don't like leading off with a Pokemon that has a 'please donk me' sign around its neck.
1-2 Sableye CG is a good play in any Blissey deck (Excavate has synergy, and it gives you an edge over other Blissey decks).
Blissvire is nice in theory, but never really works out for me. Again, bad starts.