If going by the credibility of news reports, weakest party I have seen so far is RP.
I would say SDA and it's component parties, especially PKMS.
I strongly disagree with even the existence of PKMS as a party in the first place, do you think a race based party has any place in multi racial Singapore, especially so when this party actually was the Singapore branch of UMNO here before the seperation, they also attend UMNO's general assembly up north as "special observers" .....
Note also that they were actually allied with NSP and SPP before both of these parties withdrew from the alliance, leaving two small orphans in bed together ... which just goes to show that most of the opposition parties operate out of political interest, not national interest, they'd get into bed with anyone, to follow the adage "the enemy of my enemy is my friend, nevermind who" ....
there is a saying " never cut off your own penis, just to spite a particular CB" .... so open your eyes wide when these fellows come to town.