Indubitably

Adverb from the adjective indubitable - impossible to doubt, unquestionable.
From the Latin indubitabilis, in 'not' + dubitare 'to doubt'.

This word is truly excellent in sound (just say it, it's great) and meaning but its main selling point is the fact that it is by far superior to doubtlessly. I often ask myself the question: 'why would anyone use doubtlessly when you could use indubitably?' Even the natural pronunciation of 'indubitably' makes it sound superior in the same way that a gentleman with a monocle wearing a top-hat and tails sounds superior to anyone from Essex. In fact, having entered the word into Google, the quantity of monocles and top hats in the image results is astounding. It gets the vote of Sherlock Holmes (I asked him) so we should all use it more.