The best safe haven would be a hilltop compound specifically designed for ultimate security and sustainability:
Elevation is key for defense. Its quite difficult to infiltrate a facility up a steep incline. Especially for zombies. Alternate between dirt berms and high fencing with barbed/ridged tops (concertina wire is your best friend).
The entrance and exit should be kept simple. A single driveway with a sturdy gate is the way to go here. Cattle guards also provide a solid brace to keep said gate closed. If sturdy metal poles are used, your entrance is a lot more secure.
Its all about the long haul when concerning sustainability. Plant a garden and compost everything. Fruit stands can provide essential vitamins year-round if you preserve what you don’t eat right away. Windmills and solar cells can generate electricity, and a barn, even a makeshift one, can house animals for meat/dairy needs. Keep wells clean, store wood for both heat and cooking, and place a cesspool off the property for proper waste disposal. The Zeta virus sucks, but so do skin infections.
Atop a hill, you have the advantage of, well, vantage. But having a watchtower or two is a fantastic idea, and also is installing floodlights so you can monitor what’s going down on the property during nighttime/bad weather/both.
If things get really gnarly, a buried school bus makes for a great shelter. A single escape tunnel should be dug facing the back so you can make a safe and hidden run for it along with your group (the back doors of said bus should already be removed, obviously). Camouflage and reinforce the exit point as much as possible.
All this would make for, in my opinion, as close to a perfect shelter/safe haven as you can get in all reality (besides a legit fortress, of course, but even those could benefit from a lot of my ideas presented here).
So, yeah. There’s my two cents on the safe haven portion of the conversation. Seeing some of these in-game would be sick, honestly.