This is incredibly corny but Crash will always be their mascot in my heart. Doesn't matter if it's multiplat nowadays.
Speaking objectively though, I think Sackboy had a chance but the fact that LBP3 didn't make enough of an impact burned that bridge. Nowadays, I think the closest character they have to a mascot is Kratos. Nathan Drake is of course very popular but I believe his fanbase isn't that large worldwide. Most people love the Uncharted games but don't care that much about him. GoW on the other hand has always shined the brightest due to Kratos' memorable design and personality which used to be really two-dimensional but thankfully they corrected that with the new PS4 game.