Voigtländer has several fast M-Mount lenses in the line-up: The Nokton with f/1.0 and the Nokton f/1.5 appear to be interesting alternatives to Leica’s similarly speced, but far more expensive Noctilux and Summilux lenses. Part 27 of our M Files series reviews the two Voigtländers in this truly classic focal length.
Lumix and Sigma L-Mount prime lenses offer superb optical performance in a cost-effective package. You can't really go wrong, whichever model you choose.
What is more important for a zoom lens: range or aperture? Read a few answers to these old questions - and a suggestion that opens even more options...