Depends who you ask: the New Democrats, whose 103-seat triumph in 2011 vaulted them to official Opposition status, or the third-party Liberals, who claim their 35-member caucus has been more effective at holding the government to account.
The two parties have been battling over the title of most effective opposition throughout the fall sitting of Parliament; both frequently claim to have led the charge on various issues and accuse the other of dropping the ball.
The ongoing feud was on display again Wednesday as both parties offered their assessments of the fall sitting in anticipation of an imminent, lengthy Christmas break.
They were united in their condemnation of the Harper government's alleged arrogance, incompetence and malfeasance. But before long, they were taking shots at one another as each claimed bragging rights for holding the government to account.
From behind a podium festooned with a large sign proclaiming him "The Official Opposition," lest anyone forget who beat whom in the last election, NDP Leader Tom Mulcair declared there is only one alternative to the Conservatives.
"The next election is going to be about a clear choice, a clear choice between the Conservatives — who will make people work two more years to get their old age security, who have scrapped environmental legislation, who are going to go after free, universal, public medical care — and the NDP, the only party that's standing up to them and offers an alternative," Mulcair said.
He dismissed suggestions the opposition parties should be teaming up to defeat the government.
"My No. 1 job as leader of the NDP is to rally all progressive forces in Canada under the NDP banner and defeat Stephen Harper in 2015," Mulcair said, to applause from New Democrat MPs gathered behind him.
Moments later, interim Liberal leader Bob Rae and his House leader, Dominic LeBlanc, offered a counter assessment of the session, in which the tiny 35-member Liberal caucus featured as the primary alternative to the Tories.
"We have been an effective opposition. We have stuck to issues that are important to Canadians," LeBlanc asserted.
He maintained the Liberals were "the only party that consistently and frequently raised questions" about teenager Ashley Smith's death in an Ontario prison. And he said they led the charge against the F-35 stealth fighter jet procurement fiasco, while "the NDP were late to the party."
Rae bristled at suggestions that the relationship between Liberals and New Democrats seems to be growing more hostile, even as many progressive Canadians are looking for co-operation to defeat the Conservatives.
"It's nonsensical to think that the only way you can ever defeat the Conservatives is to have two parties in the House that are fighting each other like cats and dogs," Rae said.
A three-party system gives Canadians greater choice, he added, neatly excluding the Green party and Bloc Quebecois as influential players.
Rae said New Democrats — and many pundits — bought "the fantasy" that the Liberal party was "done for" after the 2011 election. But, he said, they've been proved wrong.
"I know that Mr. Mulcair ... suffers from this almost total delusion that if he says over and over again that the Liberal party doesn't exist that maybe it'll disappear. Well, I'm sorry, that's not how the world works.
"He can pretend that we're not there but we are there and we're doing a good job, we're doing an effective job. Sometimes, he even borrows my lines."