A good opposition MP is one who can use his power base to further the Opposition Cause.
Period.
This is because there may be a visible difference in the quality of opposition wards vis-a-vis PAP wards. Thus the opposition MP need to justify his residents' faith by being a catalyst towards a two-party system or at the extreme least, one-third majority in Parliament.
25 years imples, frankly, an ineffective catalyst.
A good PAP MP is one who is willing to sit in coffee shops, listen and help.
One who is willing to walk the blocks and go up the stairs to see if the Town Council is not doing its job.
One who goes beyond the Meet-The-People session and feedback from the grassroots organizations to actually meeting the people informally and encouraging informal feedback.