Build 2007 and 2008 are optimal because of changes introduced in later versions I list below

Build 2012 - Reduced damage on star of torment soul mode slightly. Build 2046 - Weapon system changed to be frame-rate independent. Build 2055 - RT Waterfall demo level added to main game (Lost Falls) Build 2172 - Enemy counts on most level fixed (all the other versions also fix some spawns but this is the version that should be used for 100% runs)

Earlier versions are Early Access so they arent allowed on the leaderboard unless it is an IL that is not advantaged on the Early Access version (for example 100% runs on a EA level that has less enemies than in the full release etc.)