Modding the original Xbox is best done Splinter Cell (2002), the reason is because the games MechAssault and 007: Agent Under Fire require a specific disc. On the contrary, Splinter Cell can be used whether it's black label, platinum hits. In the PAL regions the original black label version can be used as well as the classics edition. It has also sold 3 million copies according to Wikipedia. *must be the original Splinter Cell, no sequel. Keep in mind that the region whether NTSC or PAL must be accompanied by softmod files that are specifically for the NTSC version of the game or the PAL version.
The other two games to soft mod the original Xbox with are 007: Agent Under Fire and MechAssault. If you do plan to softmod the original Xbox with 007: Agent Under Fire, the disc must either be the original version or Platinum Hits. If Platinum Hits then the disc must have this serial number written on the disc 1448517.
If you plan to softmod the original Xbox with MechAssault, the only compatible one has the serial number MS02301L written on the discs inner ring. Platinum Hits and the "Not For Resale" version will not work.
UPDATE: Recently Tony Hawk's Pro Skater 4 is now exploitable, in other words THPS 4 is now on the list of games that can be used to softmod the original Xbox. Both the original and/or the classics/platinum hits.