Loch Lubnaig near Callander, mostly because if we are heading into the mountains, it’s the first real bit of wild looking countryside you come to. Usually mirror smooth due to the high hills surrounding it, it has a wide beach access and a generous shallow run in.
Peat water is also great because:
1) It tends to hold its temperature better
2) Your visibility is reduced so you can’t see the creatures you’re sharing your dip with!