I'm not a coder or IT guy - just someone who spent too many hours on a flash game, so all of this ain't technical analysis of the game. And obviously, these are just my opinions, so feel free to express your opinion in the comments!

There are 2 main types of stats in the game, with Attributes being buffs applied generally to a specific group of skills. None of the Attributes are truly the best (except maybe Luck), but some are definitely better than others for a couple of reasons:
- Many of the specific Skills have little to no effect on gameplay, even with buffs from books.
- Some Skills only need to reach a certain level and/or don’t come into play as often as you might think.

When leveling up in-game, it’s highly recommended you follow this general rule. Details will be further explained in the Skills section.
High priority: Luck & Precision.
This should be obvious to any veteran: shooting plays a big part in the game, and so does Luck in giving you a chance to obtain high-quality in-game items of all kinds. Speaking of which, I’m planning to make a tier list of them in the future.
Mid priority: Strength.
Increases your carry strength, which is just really good overall. However, you have the option to either max it out pre-game at 10 to save yourself from sorting all the junk, or wait until later when you have more weapons and items in your backpack.
Low priority: Intelligence & Endurance.
Depending on how you play the game, this may come as a surprise, as objectively, ¾ Intel stats & max HP increases are generally pretty beneficial. However, many Skills of these Attributes can be ignored for further reasons below.

This is the real meat of the game: the skill system.
For this part, I’ll just go through which stats are good, which are bad, and which just are straight up meaningless to invest points in (in no particular order).

These two determine the max damage of your respective melee weapon types, with lower B&B stats decreasing it by a few points and vice versa. Investing in them or not barely makes any difference when you also take Time to Kill into account.
More specifically, with strong enough weapons, you can (usually) one-shot Commons, and two-shot Fatties. For weaker ones, it's usually the difference between having to land one more or one less total hits. Because of this, you can ignore these two stats altogether.
As an example, the Sledge, a guaranteed melee weapon pick-up inthe garage next to the saferoom in Whistler’s Forest. Thanks to its high raw stats in both Damage & Knockback, it will (almost) always kill a regular Infected in 1 hit, or stun Fatties to safely kill in 2 hits, regardless of locations.

In the case of these two, they both very slightly enhance the newbie experience by a bit, until you take into account what they actually do in-game:
+ Regarding Fitness: Essential actions like using melee weapons and walking cost no stamina and are always the same speed, while other actions require either very little or no stamina at all. In addition, from my experience, buffing them only slightly improves how long you can run and shove, both of which are entirely optional in beating the game. (Altho it might be good for speedrunning, I suppose.)
+ Regarding Survival: Requirements for food and sleep are extremely minimal, especially when food items and safehouses are just kind of everywhere. Plus, damage resistance can be partly negated with the help of a “meat shield” companion: infinite ammo, effectively infinite health as u can continuously revive them, etc.. Just watch out for explosives. Ouch.

Like a former guide writer said, ¾ of these stats are pretty based. But in my opinion, it may be worse than what people give them credit for.

Smarts give benefits towards extra XP and stat points earned from any source, like getting kills, opening containers, reading books, along with getting cheaper prices from the Pawn Shop. These do come with a few caveats, however:
- The amount of money you can obtain from containers (once you know the various lucrative money locations) can easily reach thousands around mid-game. When combined with great enough drops on weapon and helmet/armor if you choose to put points in Luck (including guaranteed ones from many containers), you can simply ignore most store items and stock up on ammo instead. Because of this, saving money is barely an issue.
- Investing ~10 levels worth of stat points onto it, just so you can get a few extra benefits from books and XP is not worth the trade. Not to mention the uselessness of some books because of the kind of stats they buff that I previously went through, and how leveling up with low Smarts can be just as fast thanks to Achievement XP.

Security is pretty straightforward: higher Security stats = better Force Lock rate & the ability to unlock Hard locks at 20, and Very Hard locks at 35. As you can probably tell, Force Lock is optional (altho there are a few achievements for it), it does not affect the unlock mini-game difficulty at all (which I think it should), and the entire game only has 2 Very Hard locks. Spoilers: one in Hank’s apartment only has a revolver and some cash, while the other in Whistler's Forest even has a key to open!
Tl;dr: just set it to 20, or 35 for an Achievement run.

First Aid is also rather simple: higher F.A stats = more health from HP items. Sharing the same issue as Survival and Hunger, the number of these items in the game are also a whole lot, and you can even buy Medkits in the Pawn Shop! Besides, the game even pauses when you’re sorting your inventory, so you can even heal with a bunch of zombies around. If you use your “meat shield”, play wisely, or just cheese the game, this stat shouldn't matter that much.

Searching is by far the hardest one to understand: RNG stats means it’s too random to pin down how it works, go figure. Speaking from experience, it does the first half of its description quite well (Reducing junk found), but it is hard to determine whether the other (Increase no. of items found) goes hand-in-hand with Luck or not. Because of this, it's a bit of a "gamble" stat. My guess is that it adds 1-2 extra Luck items when found, and that’s it. Invest in this one at your own risk.

These stats are pretty cut-n-dry: better stats mean better accuracy on a specific weapon type, and that’s it. Of course, some weapons are better than others, but the Long Guns & Automatics take the cake all thanks to a lil’ friend called RNG.
Single-fire weapon options - Pistols (and revolvers, with the exception being the burst Venom MP93R), Long Guns, and Special Weapons all require a lot of clicking, which could be circumvented by an auto-clicker. However, most of them suffer the issue of inaccuracy and/or single-target damage that can make them incredibly frustrating to use. At least the high-damage Sniper Rifles & Shotguns also have the ability to penetrate through several enemies, so they’re still pretty good.
Explosive Weapons don’t count, of course, thanks to their AoE (Area of Effect) damage, but they’re called “Special” for a reason: rare ammo types, only drops at certain containers, but at least the shots are usually guaranteed to land as a result. So, just ignore the S.W stat.
The Automatics still suffer from RNG, of course, but to a lesser degree. You gotta land something from spraying bullets everywhere, right? And also - penetration.
As a side result of this, objectively, .9mm and .45 ACP ammo types would be better spent on SMGs instead of handguns. Altho, don’t let this discourage you from using your favorite guns or challenge yourself

I think that’s about everything. Again, most of this is simply my opinion after spending enough time in the game to (hopefully) understand most of the game mechanics. Gotta hand it to the devs, tho, it’s a flash game with 2 years of development, so the fact that this guide even exists is a true testament to its intricacies (or lack thereof, in some aspects).