Though received luke warm upon its initial release, On Her Majesty's Secret Service is now oftentimes considered the best film in the series. And it's my favourite.
It's an extremely faithful rendition of Ian Fleming's superb source novel(my favourite Bond novel as well) and is basically rendered directly to the screen with very few changes. It's perhaps even more faithful than 2005's Casino Royale was to its source book.
The movie stars George Lazenby as Bond in his only outing. He does a good job of playing the character faithfully to the novel, and although Connery will always be the best, he compares favorably at least to Brosnan and Dalton in particular.
Diana Rigg brings Teresa Bond to the screen with a strong female portrayal(because modern movies aren't the only films to feature strong women). Telly Sevelas brings a grounded menacing and more realistic Blofeld to the big screen, with his Chemical Warfare plot being one of the more unsettling villainous plots in a Bond film(and straight from the book!). He delivers the most sinister portrayal of the character even out-classing Pleasance and modern actor Christopher Waltz.
The movie delibrately starts slow, showing Bond in full spy-mode slowly tracking Blofeld down and falling in love with Tracy, before the arrival at the main setting of Piz Gloria. From there it slowly switches into high gear, delivering a continuous supply of highly entertaining high altitude action set-pieces each more spectacular than the last one. From the first Ski Chase, the spectacular Rallycar chase sequence, the second ski chase involving the massive avalanche, the helicopter assault on Piz Gloria, and the sensational final bobsled chase sequence. It is all pure action joy, and the most consistent James Bond film ever.
The movie ends on a famous dramatic note with a heartbreaking final scene in which a certain character gets shot dead, leaving Bond devastated. All of it taken faithfully from Fleming's book.
It's the greatest epic action film the series ever produced.