A defacto "standard" - well worth having.
http://www.dpreview.com/lensreviews/canon_50_1p8_ii_c16
The other one to consider is the 85mm f1.8 prime
http://www.thephoblographer.com/2011...5mm-f1-8-lens/
(This is being used with a Star Analyser "objective grating" to record stellar spectra down to 8mag)