Shoutouts to SoulEchelon, Undertaker Grim, et al for their work on the Shop List and stat boosts/miscellaneous functions for all the items in the game.

For those who don't know exactly what stat affects what, here's the breakdown:

ATK - Damage dealt by special attacks
WPN - Damage dealt with weapons (or enemies, if you're holding them)
THR - Damage dealt by thrown weapons/enemies
AGI - Walk Speed/Jump Height
DEF - How much damage you can faceblock
STR - Damage dealt by normal attacks
WLP - How much extra HP you can store
STA - Your Health. If it hits 0, and you have no WLP available, you're D-E-A-D
ENG - Your Guard Gauge (Bar below your character portrait). More ENG means more blocking before your guard is broken.
SPC - Special Power. More SPC means more Glen Blustery Kick spam.

It's fairly common to grind for Edamame and Octopus Rolls (Saunas are a big given) in the early-game. But once you hit Act 2 (when Rollingwood and Merville are accessible), your options for stat increasing expand tremendously. And with everything you can buy in the game, it's easy to ask yourself what the best bang for your buck is. If you want an even spread of stat gains, then Edamame and Octopus are often the go-to. But if you want to focus on a single stat (with the exception of ATK/SPC, but I'll get to that), the items of choice change.

For Act 1 only:
ATK - Dry Beef Stick (Wee-Mart, 24hr) $2.75, +1
WPN - Ghost Chili (Carnita's Way, Day Only) $7.60, +2
THR - Salad Burrito (Carnita's Way, Day Only) $6.50, +2
AGI - Arachnid Guy (Geoffrey's Comics, Day Only) $16.10, +8
DEF - Major America (Geoffrey's Comics, Day Only) $21.25, +10
STR - Pollo Taco (Carnita's Way, Day Only) $3.30, +1
WLP - Street Fighting (Bookface, Day Only) $13.50, +30
STA (Heal) - Merv Rings (Merv's Burger's, 24hr) $4.45 (portable), +40 or Wanna Sauna $5.55 (overall), +60
Max STA - Wanna Sauna $5.55, +15
ENG - Chicken Burger (Chicken Burger, 24hr) $7, +2
SPC - Dry Beef Stick (Wee-Mart, 24hr) $2.75, +1

For Act 2 and beyond:
ATK - Dry Beef Stick (Wee-Mart, 24hr) $2.75, +1
WPN - Whips Vs Vampires (Warp Zone, Day Only) $22.15, +15
THR - Ackwrite (Stubash Records, Night Only) $14, +7
AGI - Speedo the Wombat (Warp Zone, Day Only) $13.15, +7
DEF - Major America (Geoffrey's Comics, Day Only) $21.25, +10
STR - Pollo Taco (Carnita's Way, Day Only) $3.30, +1
WLP - Street Fighting (Bookface, Day Only) $13.50, +30
STA (Heal)- Merv Rings (Merv's Burger's, 24hr) $4.45 (portable), +40 or Wanna Sauna $5.55 (overall), +60
Max STA - Wanna Sauna $5.55, +15
ENG - Chicken Burger (Chicken Burger, 24hr) $7, +2
SPC - Dry Beef Stick (Wee-Mart, 24hr) $2.75, +1

These are the items that give you the most value per point. Considering how grind-intensive this game can be, it's important to factor cost-efficiency into your stat leveling.

IMPORTANT NOTE: Shop availability becomes 24hr across the entire city once you hit Act 3 (after beating Wes).

Hope this list helps you in your quest to max out your characters, and feedback is greatly appreciated!

Special thanks to Phil for recommending I list shop availability, Lord_Swarz for the Stat description suggestion, and Crumpet for requesting a list of Act 1 items, since some of the better stuff isn't available until Rollingwood et al are available.