what are you going to use your camera for? that's step #1 before dropping big bucks on prime glass. for my D60 i have a 50mm f/1.8, 100mm f/2.0, & a 28-135mm 3.5-5.6 IS.
the 50mm f/1.8 -- a great, sharp, fast, inexpensive, lightweight lens that becomes an 80mm on the D60/30.
the 100mm f/2.0 -- again, wonderful, sharp, & fast, becomes 160mm on the D60/30. this lens creates excellent bokeh (background blur) for outdoor environmental portraits while "flattening" the perspective for a more flattering look. close focus distance isn't comparable to the 100mm f/2.8 macro, but it depends what you use it for.
if you're looking for a wide angle prime, consider the 20mm f/2.8. it becomes a 32mm and is >3x cheaper than the 16-35 f/2.8 zoom.
longer applications, the 300/400/600mm L primes are incredible. i've never used one but have seen plenty of gorgeous shots taken with them.
don't rule out zoom lenses either. many of the "consumer" zooms preform VERY well, especially if you stop them down to f/8 or so... seems to be a sweet spot w/ many canon lenses. i have the 28-135mm IS & love it, despite people's complaints here. it becomes a 44-215mm zoom & is very sharp if used well... plus the IS is well worth the $$.
i work for a pro who shoots portraits w/ a D30 and a 28-105mm stopped down to f/8 & they're very sharp... i've seen perfect prints up to 20x30" he's made using this gear... & you can find that lens for $200 @ B&H (gray).
so, back to the original question... what are you shooting the most?