The weakest was certainly "Sharpe's Justice" (#13).
"Sharpe's Waterloo" (#14) airs tomorrow (Sat.) at 7pm Eastern, on BBC America.
IMO It's a good episode. Not great, but good.
I've read all the books and seen about half the films. As is often the case, I think the books are much better. Furthermore, you can tell they've skimped on production costs. We're talking about the clash of battalions, divisions and armies...and half the time you're lucky to see 50 people on the screen. The real advantage held by the books is that they really provide the history of the Peninsula War as well as explain the men, weapons, tactics and strategy involved. Plus, Sharpe and Harper are great characters.