Kopitar's been the best offensive AND defensive forward on the Kings for over a decade. If it weren't for Bergeron being in the conversation for best defensive forward ever, there'd be a lot more attention on Kopitar there.
Led the playoffs in scoring twice, has a 92 point season to his credit, his average production is 73 points...and you can use his PPG because while Bergeron has never played all 82 games in a season, Kopitar has done it 6 times, and played 97% of his possible games in the NHL while often leading the NHL in forward minutes played in all situations. The guy is an absolute horse, old reliable.
He's obviously got great chemistry in Brown and previous has with Justin Williams, Ryan Smyth, and one playoff run of Gaborik, but other than that, his wingers have been guys like Kyle Calder, Patrick O'Sullivan, Dwight King, etc., which pales in comparison to what Bergeron has played with.
I do think Bergeron's offensive abilities get undersold sometimes for a few reasons (i.e. look what he did even with Crosby, guy's brain is massive, he can play at anyone's level), but Kopitar has been almost completely overlooked in comparison.