What, no love for the 75-300 EF III? :-)
After decades of FD-mount Canons, I tried a Rebel G (EOS 500N) just for fun - and it is fun - but the only lens I could get with it at the time was the 75-300. It's crude and rough to use, but actually decent optically.
For must-have, I'd say 50/1.8 - any version. They're inexpensive, reasonably good optics, fast, lightweight. Mine's a II, slow to focus and noisy, but it seems to nail the focus and produce nice images - as good as the 50/1.4 on my Elan 7NE.
I've yet to use my 35/2 IS. Pretty lens, though.
"Great photography is about the visual effect upon the viewer, not sharpness." - Stephen Gandy, Cameraquest