Which is the least stable trihalide of nitrogen?
Nitrogen triiodide $\ce{NI3}$ is an extremely sensitive explosive that explodes with the slightest touch when dry.  

Nitrogen trifluoride $\ce{NF3}$ is so stable that it is a greenhouse gas global warming concern, with lifetime in the atmosphere of hundreds of years.

$\ce{NI3}$ is unstable due to steric strain as you are saying.