i'm in your conscience
I think a lot of you are missing the point of the theme being summer Pokemon, tbh. It's fine if you prefer something in game, but you should also consider the theme. Granted, they're all summery Pokemon in my eyes (and to those that helped me decide on the three in the poll this week), but it seems like hardly anyone has taken the theme into consideration.
Anyway, while I think all three fit the criteria (Butterfree and Lapras are even in the Southern Islands mini-set, which is very summery!), I'm going to go with Bellossom. It just makes me wanna go to Hawaii for the summer and forget all my troubles...which would be great right about now, haha. ;( I've used it in Sapphire for uhhh something...some even idr which and I enjoyed it. Haven't used Butterfree in a while. And Lapras I'm using in White 2 now, and I was using it in my last playthrough of SS. Definitely my MVP. But as far as the theme goes, I think Bellossom suits it best to a t.