Most of my dives have been in Brittany (west of France), near the Channel/Atlantic border.
Next time will be in April, then in summer. I am lucky that my parents have a holiday house in this area, close to several diving centers.
Cool water (April about 12C, summer about 15C), kelp, granitic rocks, rich fauna even if less coloured than tropical fishes. Biggest divable wreck in the world (except that it is broken), the supertanker Amoco Cadiz.
Not crowded, even in peak summer holidays (never saw boats fron several diving centers on the same diving site). Viz often around 4-5m, vaiable. Possibly strong flow depending on the site (huge tides).