primes - 50 1.4 is the most popular choice. 40 2.8 is the smallest (DSLR lens presently available from any manufacturer), other options which should be quite easily available are:
31 1.8, 43 1.7, 77 1.8, 70 2.8
Eh. Don't know what you should get. I have a 50 1.4 mainly because I can't get anything that fast elsewhere, and it' s pretty affordable. (only the 40 2.8 costs less).
--
I agree generally with the point that fengwei and seafone are making - that there are better options for image quality than the 18-250, but I think it's worthwhile noting that the 18-250:
1. has a filter size of 62mm and weights 430g - it's not really that large/heavy.
2. appears to be better (From the bobatkins review, and elsewhere) than the 3rd party iterations of the 18-200. However, there's probably some sample variation.
And - guys - changing lenses sucks. IT's sometimes a necessary evil, but... it still sucks.