What does "兵器谱排行榜第一位" mean/represent?
This is a Chinese pun-like joke known as 歇后语. What they have in common is two parts, where the second part is a homophone of the expected answer to the first.

In this case, "够贱" is supposed to sound like the true answer to "兵器谱排行榜第一位". That may not be obvious at first, and I think this joke was told poorly, but my guess is that "兵器谱排行榜" actually refers to the Eighteen Arms, of which there are four that are considered the "best" (四大兵器), which includes the 剑 or double-edged straight sword. So "够贱" is a homophone of "够剑", or "as good as Jian".

So a proper translation might be:


  You really are the first among the weapons - as good as Jian (so despicable).


With the joke working because "jian" sounds the same as "despicable" in Chinese.

The problems with this joke are:


There's no well-known list known as 兵器谱排行榜
Of the lists that are well known, such as the 四大兵器, although 剑 ranks highly, it doesn't rank first