The 105 is the lowest-numbered bus route to serve an airport, (LHR or LCY) and also the lowest-numbered bus to use a tunnel - 105.
The 108 uses the lowest tunnel.
The 20 is the lowest-numbered route to have never been operated by Routemasters (of either kind)
The 81 is currently the lowest-numbered route to serve a non-Oyster-enabled station (Slough), but this will become 117 (Ashford and Staines) when Slough becomes TfL-X-Liz.