DeSantis 'Scorpion' IWB. Works for my G27 and G19. 24/7 for me!
Scorpion IWB Holster
You asked as many before you and many will after you, and I'll always be recommending this one. Easy on/off, positive retention, fits all similar frame sizes of Glocks, it won't absorb sweat, it's comfortable, and it's my awesome summer carry rig under a simple t-shirt tucked or un-tucked.