Owner Zeff (in the Baratie Arc) was said to be a famous and fearsome pirate from the Grand Line. However, there has been no mention of him since then by other pirates/marines. So was he really a famous and strong pirate or was he just strong compared to normal pirates in the East Blue?
All we know about Zeff is that he was strong enough to sail his crew around the Grand Line for a year.
This was a big deal in the East Blue, where even Don Krieg couldn't last a week in the Grand Line, but doesn't tell us that much about his power.
Before the timeskip, Luffy and Zoro had been in the Grand Line for less than a year, and had both become pretty strong, so he could potentially be even much stronger than they were, but also pirates like Foxy and fake Luffy seemed to be quite relaxed in the Grand Line and weren't anywhere near as powerful, so there's a pretty big range in what strength a year in the Grand Line might get you.
It was ten years ago that Zeff travelled the Grand Line, and the only two things that really bought the Strawhats fame were defeating Rob Lucci and Luffy's actions in the Whitebeard War. Zeff might have done some impressive things, but wouldn't necessarily become super famous, and ten years is quite a while for people to forget him.
Also, having lost a leg and gotten older, it's pretty sure that he's a lot weaker than he was, and it's unlikely at best that he would be as strong as the Strawhats these days.
I think Zeff was just a great pirate in the East Blue, because as far as we know, he was one of the few pirates who made it to the Grand Line. From an actual point of view, he would be a weak pirate in my opinion. (No offense against Zeff fans ;)
After all, he was on the Grand Line for just an year.
Zeff originally made a name for himself as captain on a pirate ship. He became both great at being a chef and a pirate. His dream of finding the "All Blue" led him to the Grand Line. After one year of traveling around the Grand Line, he left that sea.
Further reading: http://onepiece.wikia.com/wiki/Zeff