There isn't a simple "best" answer, and patients react differently to different strains, but I'm happy to offer some general information.
CBD is very effective at treating anxiety and can have a sedating, relaxing effect. If your insomnia is due to anxiety or racing thoughts, a higher ratio of CBD:THC may be helpful.
THC can be energizing and uplifting for some patients and can worsen anxiety in others. Strains lower in THC may be better for insomnia. CBD also counteracts some of the effects of THC.
CBN is sedating, and CBN levels increase as marijuana ages, so you might consider older cannabis!
In general, patients find that indica strains tend to work better for insomnia than sativa strains.
Also, while vaporizing is faster-acting, edible cannabis is longer-acting; so an edible indica might be the solution for maintaining sleep and comfort for a full 6-8 hour rest period.
You might try: Alta California's Botanicals: Insomnia Relief Tincture. It has a good percentage of CBN, which is good for sleep.