I would say, in order of priority:
1. Secure parking
2. Cover the parking
3. Showers
4. Full locker room

If your company already has locker rooms with showers, is there a janitorial closet nearby that could be made into bike storage? That's about as good as it gets, and most of the places I've worked have had at least one janitor closet that has fallen out of use.

Good, secure parking right at - or even inside - the door can make a big difference for a lot of people. With my knees, I'd rather pedal 3-5 miles and park at the door than drive and have to walk 100 yards or more from the car to the door. My wife bike commutes to her teaching job from time to time because the university has notoriously bad parking but the building she needs to get to has a bike rack at each end within ten yards of the door. Right at a mile door-to-door takes just about as long either way due to the search for a parking space, and it's not far enough for the air conditioner in the car to make much progress on 105F heat anyway.