The most desirable here will vary from person to person, and correctness on which language rules they subscribe to. I can say that "I always will" sounds more fluid to me, but the added "do" provides an affirmation which is important to take into account when considering desirability. After all, would you prefer someone to say "I love you", or "I do love you"? Particularly when considering the circumstances in which each are often said, I suggest most might say "I do".
But addressing the point of correctness in a loose sense, to say 'I will always do' is similar to the Irish version "I always will do". In the first case the emphasis is placed on the length of the action, suggesting commitment - in the second case the emphasis is placed on the affirmative, and is used as a common wordtype-pair - I cannot say either are incorrect. Ultimately what is important here is the added affirmation behind the word "do", and while it is implied that one does love when they say that they love, the emphasis on the enduring nature of her love, via word order, is endearing.
In an effort to compromise you might agree on something like "I do love you and I always will".