If not so much budget constraints then 85 f/1.8($550)
A bit more money then 17-55 IS f/2.8 ($1200) for crop and 24-70 f/2.8 ($2000) for full frame
If you take only faces then 135 L (check price thread)
If you take group photos then 35 f/2 ($430)
You can have the above at f/1.4 also, better performance at a higher cost
Beside Canon, Sigma f/1.4 are also great and many consider those better than Canon counterparts.
So you see... you are spoilt with choices.... and the good part... these lenses does not changes every year like ur bodies!!! some of the lenses are here for more than 10 years