Well if you just mean Defence (as in physical defence) then Shuckle has the highest at 230. Steelix is probably better though, as it has 200, and actually has the health to take some hits (Shuckle only has 20 base health - Steelix has 75).
If you mean the Defences in general, then Shuckle still has the highest with 230 Special Defence as well as its 230 Defence, but again its health lets it down.
Physical Defence only, I think Steelix is best. Special Defence only I think Blissey is best (130 Special Defence combined with its 255 base health).
However, for a mixed wall my final answer is ... Bastiodon! It has 168 Defence, 138 Special Defence, and 80 HP, so is the best at taking both physical and Special attacks, although its typing does let it down quite a bit, as it is 4x weak to both Fighting and Ground, which are two very common attacking types.
