Worst line from Obama's speech: "And those of us who care deeply about programs like Medicare must embrace the need for modest reforms." It doesn't meed modest reforms; it needs substantial reforms. Also, Obama's been an opponent of modest reforms.
Worst line from Rubio's response: "When we point out that no matter how many job-killing laws we pass, our government can’t control the weather – he accuses us of wanting dirty water and dirty air." As if trying to constrain carbon emissions were some kind of mad-scientist scheme.
Worst line from Rand Paul's speech, I don't know (having not seen it or read a transcript yet).
UPDATE: I've now read Paul's speech. Worst lines (a tie, I think): "To begin with, we absolutely must pass a Balanced Budget Amendment to the Constitution!" and "The value of the dollar shrinks with each new day." A BBA probably would be ineffective at controlling spending, more likely setting the stage for future fiscal-cliff-type crises and gimmicks. And inflation is not a significant problem at present, so pretending otherwise is at best a distraction. On the plus side, Paul made some cogent points about executive overreach, and didn't (bother to) make denialist comments on global warming.