The Panasonic Leica DG Vario-Elmarit 50-200mm is an impressive value and Richard Wong finds almost nothing to fault with the lens. It even works well with the 1.4x and 2.0x teleconverters.
Leica DG Vario-Elmarit 50-200mm: B&H Photo / Amazon / Adorama
Panasonic DMW-TC20 2x TC: B&H Photo / Amazon / Adorama
Panasonic DMW-TC14 1.4x TC: B&H Photo