I'll try keep the prices relatively reasonable here, despite it not being 'an issue'. None of the following lenses are absurdly priced, but are top class lenses IMHO.
1. Sounds like you want a wide-angle for this, if you're doing cityscapes etc. Try the Canon 10-22, the Sigma 10-20, or the Tokina 12-24; use a tripod, remote shutter release etc. Great for landscapes too. For the moon you'll need something longer (see 2), and if you're feeling really adventurous for the architecture, try a tilt-shift lens! Canon 17-40L would also be a nice choice here, but a bit pointless if you get the 17-55 IS (see 4).
2. If you want a zoom; Canon 100-400 L IS. Otherwise, if you have the money (and by this I mean a LOT more money), one of the long primes, like the one Dave suggested.
3. EF 100mm f/2.8 Macro would be a good choice indeed!
4. Canon 17-55 IS; or the 24-105 f/4 L IS. Both are awesome; the 17-55 has a nicer range for a cropped sensor like the XTi's, but won't fit on a full frame camera and doesn't have L build. The 24-105 gives you more reach, L build quality, and if you ever upgrade to full frame, it'll fit and give you a sweet focal length range. If you don't want to break the bank, the Sigma 17-70 and the Tamron 17-50 are also fabulous choices.
With these lenses, I'd say you're pretty much covered for almost every type of shooting. You might wanna look at some fast primes too, like the 85mm 1.8/1.2L, the 50mm 1.2L/1.4/1.8, Sigma 30mm f/1.4, Canon 35mm L, 135 f/2 L (FANTASTIC lens).
My ideal lens lineup (and still reasonably priced) would probably be this sort of lineup. Throw in maybe a 70-200 f/2.8 IS L and you're set for life!