I've been in a similar situation as a co-author in a paper where the corresponding and the senior author are the same. In this case, this author listed herself as the first author (since she made most of the work and wrote the paper) and the remaining co-authors were listed alphabetically. An important detail is that some journals allow or even require that you specify the contributions of each author.
I have been in a similar situation recently (I did about 70% of the work) and in the end I decided to go as last author and put the persons that contributed second most (about 15%) as first author.
The decision was mainly based on what would benefit me personally more and at my current career stage and it seemed more important to me to get the number of senior authorships up (n=4) than the number of first authorships (n=16) - especially for the tenure-track evaluation ... and it also made the fist author very happy.
I would disagree with the other posters: you should be last author. Reasons:
Being last author is more prestigious/important.
Students are expected to be first author. If any other authors are your student, if you make yourself first author, people who are not familiar with the situation may assume you have stolen the first authorship from someone you have power over.