Well, I just checked the SBS Transit bus guide, and...
Route 354 - 4 fare stages from Pasir Ris to Downtown East and
back
Route 284 - 3 fare stages from Clementi to Clementi Ave 4 and
back
Route 103 (portion of former route 378 ) - 2 fare stages from
former Jalan Kayu Terminal to Seletar Camp gate and back
You know who wins hands down
.