Not counting PMs like Clark, Campbell, Turner, etc., who weren't in long enough to judge (and Justin Trudeau, for that matter), Harper has to rank at or near the bottom of any rankings, right? His competition, basically, is:
Macdonald
Mackenzie
Laurier
Borden
Mackenzie King
Bennett
St. Laurent
Diefenbaker
Pearson
Pierre Trudeau
Mulroney
Chretien
Maybe Meighen and Martin, depending on whether you want to count their shorter terms (I could go either way, though let's say Martin counts, since his party was actually elected)
Macdonald, Laurier, Borden, Mackenzie King, St. Laurent, Pearson, Trudeau, Mulroney, and Chretien were all unquestionably better.
Mackenzie signed our first free trade deal with the US, and introduced the secret ballot in our elections, so I'd put him above Harper -- who arguably harmed our relationship with the US, and wasn't so great for electoral democracy -- for those reasons alone.
Bennett was probably worse, though I think he was unquestionably a better person, plus he was done in by circumstances well beyond his control.
Diefenbaker was kind of a kook, but he also did a few good things (the first Bill of Rights, the first female and ethnic cabinet ministers, full suffrage for all Canadians), so I'd put him above Harper.
Martin was indecisive enough to not cause any major problems, but he also put ambition above country and, in the process, paved the way for Harper, so I think that knocks him down near the bottom of any ranking.
So...he's what, 12th out of 14, at best? May he fade into history and never be heard from again.