The 250GB drive that shipped with my G5 is a Maxtor, though I wish it wasn't (noisy, hot, and from semi-anecdotal evidence not terribly reliable).
If you're not going to be anal about matching your volumes (in which case you should wait and see what Apple sends you, since I don't think they're consistent--depends on whatever is cheap at the time), I'd personally recommend Hitachi as well.
I bought a Hitachi for my main drive, and at this point they seem to be the best choice for SATA; they're priced reasonably, they're the fastest thing outside of a Raptor (in fact, they're nearly as fast as a first gen Raptor in most benchmarks), they're quiet, they have a 3 year warranty, and I haven't heard any serious complaints about reliability since the 75GXP debacle a couple years ago.
I probably would've preferred a Seagate, since they're very quiet, tend to run cool, and have a very good reliability record, but for some bizare reason their SATA drives only have a 1 year warranty. Embarassing, if you ask me, especially since the high end non-SATA versions of the exact same model have a 3 year warranty.