Is there a single word or idiom for " the first thing to consider is"?
I think priority may fit: (from TFD)
- Precedence, especially established by order of importance or urgency.
- A preceding or coming earlier in time.
- Something afforded or deserving prior attention.
or the expression: 'at the top of the list':
- If something is at the top of the list, it is of highest priority, most important, most urgent, or the next in one's line of attention.
First off and first of all are other common expressions.
A rather unusual (importance-)emphasising pragmatic marker is imperatively.
Imperatively, we MUST peel off probable Republican votes.
(Dennis Macek speaking to Dave Domina) on Facebook
Most importantly is rather more common, but I think that 'importantly' on its own is a rather rare pragmatic usage.
The idiom front burner or to be on the front burner means
to be of the hiɡhest priority: It helps to put an important issue back on the front burner. It's on my front burner.
Adv : Primarily, means in the beginning
Topmost fits the description in the question. Another word may be foremost.
Foremost (Collins Dictionary) The foremost thing or person in a group is the most important or best.