For ultimate IQ seeker: all prime lens
For starters: the kit lens
For people who wants convenience and light load (me): 18-250mm
For the little voice in my head: any lens I don't yet have
Seriously tho, it is widely accepted here the FA50 is a must have lens in most scenario. The wide aperture and small size means it always leaves you the option to have a very fast lens without significant addition to the load. Not to mention it is still in production so there's no need to fight for it in fleabay's bidding war. I just got myself the FA50 and I have to agree to all of this.