I think this thread is great. I loved the RHS gardens at Wisley - truly spectacular & Sissinghurst, near where I used to live, is lovely especially for me the White Garden.
This 'garden' display is well worth a visit if you're visiting the Charente Maritime department of SW France. It's called Jardins du Monde (Gardens of the World) & that's exactly what's displayed as you stroll from one 'country' to another. There's also a tropical house as well as a wonderful bonsai room. It looks like they've added more for children since I last visited but it doesn't matter what time of year it is as there's always something lovely to see. Sorry the links is in French but if you access 'Visite Virtuelle' it'll give you an overview of the gardens.
Another unmissable garden if you're visiting France is the Palace of Versailles. Be prepared to queue, even if you buy tickets in advance you still have to go through the security checks.The whole place is truly stunning - wear VERY comfy shoes!
Happy Summer !!