Mirrorlessons is impressed with the Panasonic Leica DG 8-18mm f/2.8-4.0, but the Olympus M.Zuiko 7-14mm f2.8 Pro is also a good option if you prefer Olympus lenses.
Panasonic Leica 8-18 mm: B&H Photo / Amazon / Adorama
Panasonic GH5: B&H Photo / Amazon / Adorama
Olympus M.Zuiko 7-14mm f2.8 Pro: B&H Photo / Amazon / Adorama