The classic choice in the 'old' days was 28, 50 135, not so now.
You have a R4a which unlike my camera works with lenses wider than 28mm. If I had it I would go with a 21 like the VC f4. You don't mention a budget, assuming like most of us - especially me, you don't want to spend a fortune. The VC is a pretty good lens for the money, soft at the edges but it renders tone and detail quite well and is very small. It is slow, but that's the pay off. It's on ebay often.
I have the latest Elmarit 28 and the VC 21. I use the 28 more, maybe because it's a fantastic lens, but also because of the VF frame, the finder I have is a pain so I usually just guess and maybe crop with the 21. As others point out, cropped it becomes a 24 / 28 whatever - so why not just use a 28? I'd use the 21 more often if I had a good VF.
I had the 24 Elmarit for years, but for some reason I never really loved it, so PX'd it for a new body. I think it's an in-between length, a lot bigger & heavier than the 28 for the extra angle you get and more money.
Tried out a 15mm VC the other day, plenty of colour madness but a very nice little lens too. I wish Lightroom 4 had made profiles for VC, why doesn't someone clever make them?