Winter: Some commercial stuff mostly from both VW and BMW. Purchased in bigger plastic jugs it's similar price as most supermarket antifreeze. Paid less than 10 € last time for a 5 Liter jug of BMW concentrate and it's -55°C, so I usually dilute it 50/50 at least.
Summer season: Destilled water + two drops of dish soap usually. Sometimes tap water, but I try to avoid to replenish water at gas stations as both my Mini and Porsche lack a strainer (which the GTI has). Sometimes hard to spot whether that water's clean or dirty.