If you can only have one lens and flexibility isnt important, there are only three choices:
- 35 f1.4
- 200 f1.8
- 85 f1.2
The 85 f1.2 is the best lens you can get anyone saying the contrary has never tried it. Very few people have tried it compared to other lens because it is too long on 1.6 crops and cost wayyyy too much compared to its f1.8 relative. But the difference is stelar. It is like the first time you replace the kit lens with a 50 1.8...
The 200 is also incredible and will get you press passes were ever you go, but you will never be able to do candid with it.
If you want more flexibility and have less cash, go for the 24-105ISF4. It is the best money/quality you can get for that range. Then, its the 70-200ISf4. After that, you are going into the realms of supertelephoto, but the 28-300 is known to be the best safari lens one can get and the 400 5.6 the best birding lens (when you have to walk...).
If you dont have much money, the 100mm Macro is a very good deal too but being stuff at 160mm is a bit long.
Anyways, get your credit card and buy a 85 1.2 ;-)