If you could only have one........

Did not realize a like thread was running.

Probably 50L. But there is another thread running for this.

Its an impossible question to answer for me - only one prime lens really limits one drastically to what you can shoot - especailly if one has diverse interests.

I'd be torn between a 300mm f2.8 IS L, MPE65mm f2.8 macro and the sigma 150mm f2.8 macro and heck that totally leaves out landscape work or even longer telephoto prime lenses.

The trick with choosing a good new lens is not to try and find the most popular, because you simply won't find it and even if you do it might not be the right lens for you (had I started with a 50mm prime I might not be as far as I am today).

First sit down and work out your interests, the subjects and shots that you are interested in and want to display/take. Then also consider how your current setup works with regard to those subjects - can you shoot as you wish or do you feel yourself limited in any way - or does your current gear cover you well for your subjects, but you would like to upgrade to the next level of quality?

Once you have a context for the upgrade you can then start to consider options = first try to work out a rough idea of how much you are willing to spend and then take a look at the market - or ask around the forum for ideas. Do remain open to more expensive suggestions as it might be that if you save for just a little longer you can afford to get something a lot better than what you can afford today.

On what? A crop or full frame sensor?