Well, a fast lens is nice, but a sharp lens is better.
A lens you use often, the best.
The 24-70 mm 2.8 is a nice one, the 16-35 mm 2.8 is a good one for aquariums.
I have several longer L's, but they are large and I rarely ever have a need for them.
General lens, macros and wide angle.
The...