We have all heard about the Holy Trinity primes for your "L" belivers but I have an alternative suggestion.
I propose the "Fab Four" primes for us bargan basement hunters:
a. EF 20MM f/2.8 USM at $420
b. EF 28MM f/1.8 USM at $400
c. EF 50MM f/1.4 USM at $315
d. EF 100MM f/2.0 USM at $390
Together these sum to $1,525 (as recorded at B+H) as opposed to the $4,000+ for the Holy Trinity
Of course one could substitute the 100MM f/2.8 MACRO or the 50MM f/1.8, or the 15MM f/2.8, or.....
The drill is to find four good prime lenses that cash challenged photographers could use now as they save up for the more expensive "Real Deal"