"Best" as in highest calories per dollar. I am curious and so I compile the list from the Taco Bell website. Data is retrieved on April 21, 2021 from https://www.tacobell.com/food. I excluded items that vary in calories because of options, 0-calories items, and drinks. Some specific exclusions include "My cravings box" and party packs.
Sauces are on the top of the list. But assuming you are not buying $3 worth of Spicy Ranch Sauce and drink them for 2200 calories and receiving the gift of diarrhea, the best item is the Beef Burrito ($1 for 430 calories) from the value menu. The next two that follow closely are also from the value menu -- Beefy Potato-Rito and Cheesy Bean and Rice Burrito. The first non-value menu item at the top of the list is the Hash Brown Toasted Breakfast Burrito ($1.99 for 580 calories for 291.5 calories per dollar.)
For taste, I like the Beef Burrito and the Chicken Chipotle Melt (only $1 for 190 calories because ... chicken).
| Food Item | Price | Calories | Calories per $ |
| Spicy Ranch | $0.30 | 220 Cal | 733.3 |
| Avocado Ranch Sauce | $0.35 | 240 Cal | 685.7 |
| Creamy Jalapeño Sauce | $0.35 | 230 Cal | 657.1 |
| Chipotle Sauce | $0.35 | 220 Cal | 628.6 |
| Beef Burrito | $1.00 | 430 Cal | 430.0 |
| Beefy Potato-Rito | $1.00 | 420 Cal | 420.0 |
| Cheesy Bean and Rice Burrito | $1.00 | 420 Cal | 420.0 |
| Hash Brown Toasted Breakfast Burrito | $1.99 | 580 Cal | 291.5 |
| Drinks Party Pack | $4.00 | 1120 Cal | 280.0 |
| Bean Burrito | $1.29 | 350 Cal | 271.3 |
| Chalupa Cravings Box | $5.00 | 1340 Cal | 268.0 |
| Cheesy Toasted Breakfast Burrito | $1.29 | 340 Cal | 263.6 |
| Cheesy Toasted Breakfast Burrito Potato | $1.29 | 340 Cal | 263.6 |
| Hash Brown Toasted Breakfast Burrito Combo | $4.19 | 1020 Cal | 243.4 |
| Breakfast Crunchwrap | $2.79 | 670 Cal | 240.1 |
| Spicy Potato Soft Taco | $1.00 | 240 Cal | 240.0 |
| Quesalupa Combo | $5.99 | 1360 Cal | 227.0 |
| Breakfast Crunchwrap Combo | $4.99 | 1110 Cal | 222.4 |
| Chips and Nacho Cheese Sauce | $1.00 | 220 Cal | 220.0 |
| Hash Brown Toasted Breakfast Burrito Steak | $2.69 | 570 Cal | 211.9 |
| Nachos Party Pack | $9.99 | 2050 Cal | 205.2 |
| Nachos BellGrande® Combo | $6.69 | 1350 Cal | 201.8 |
| Grande Toasted Breakfast Burrito | $2.79 | 560 Cal | 200.7 |
| Grande Toasted Breakfast Burrito Combo | $4.99 | 1000 Cal | 200.4 |
| Black Bean Quesarito | $3.19 | 630 Cal | 197.5 |
| Beefy 5-Layer Burrito | $2.49 | 490 Cal | 196.8 |
| Quesarito | $3.39 | 650 Cal | 191.7 |
| Chicken Chipotle Melt | $1.00 | 190 Cal | 190.0 |
| Cheesy Roll Up | $1.00 | 180 Cal | 180.0 |
| Cinnabon Delights® 12 Pack | $5.19 | 930 Cal | 179.2 |
| Veggie Nachos Party Pack | $9.99 | 1770 Cal | 177.2 |
| Quesalupa | $3.49 | 610 Cal | 174.8 |
| Cinnamon Twists | $1.00 | 170 Cal | 170.0 |
| Quesalupa Black Bean | $3.49 | 590 Cal | 169.1 |
| 3 Soft Tacos Combo | $5.79 | 970 Cal | 167.5 |
| Crunchwrap Supreme® Combo | $6.89 | 1120 Cal | 162.6 |
| Grande Toasted Breakfast Burrito Steak | $3.49 | 560 Cal | 160.5 |
| Chicken Quesadilla Combo | $6.89 | 1100 Cal | 159.7 |
| 3 Crunchy Tacos Combo | $5.79 | 920 Cal | 158.9 |
| Nachos BellGrande® | $4.79 | 740 Cal | 154.5 |
| Cheesy Fiesta Potatoes | $1.49 | 230 Cal | 154.4 |
| Burrito Supreme® Combo | $6.49 | 1000 Cal | 154.1 |
| 3 Soft Tacos Supreme® Combo | $6.79 | 1040 Cal | 153.2 |
| 2 Chicken Chalupas Supreme Combo | $8.39 | 1240 Cal | 147.8 |
| Cheese Quesadilla | $3.19 | 470 Cal | 147.3 |
| 3 Crunchy Tacos Supreme® Combo | $6.79 | 990 Cal | 145.8 |
| Guacamole | $0.50 | 70 Cal | 140.0 |
| Crunchwrap Supreme® | $3.79 | 530 Cal | 139.8 |
| Soft Taco | $1.29 | 180 Cal | 139.5 |
| Cheesy Gordita Crunch | $3.59 | 500 Cal | 139.3 |
| Black Bean Crunchwrap Supreme® | $3.79 | 520 Cal | 137.2 |
| Chicken Quesadilla | $3.79 | 510 Cal | 134.6 |
| Crunchy Taco | $1.29 | 170 Cal | 131.8 |
| 3 Doritos® Locos Tacos Combo | $6.99 | 920 Cal | 131.6 |
| 3 Doritos® Locos Tacos Supreme Combo | $7.59 | 990 Cal | 130.4 |
| Steak Quesadilla | $3.99 | 520 Cal | 130.3 |
| Cinnabon Delights® 2 Pack | $1.29 | 160 Cal | 124.0 |
| Hash Brown | $1.29 | 160 Cal | 124.0 |
| Soft Taco Supreme® | $1.79 | 210 Cal | 117.3 |
| Black Beans and Rice | $1.49 | 170 Cal | 114.1 |
| Chalupa Supreme | $3.19 | 350 Cal | 109.7 |
| Crunchy Taco Supreme® | $1.79 | 190 Cal | 106.1 |
| Burrito Supreme® | $3.69 | 390 Cal | 105.7 |
| Black Bean Chalupa | $3.29 | 330 Cal | 100.3 |
| Nacho Cheese Sauce | $0.65 | 60 Cal | 92.3 |
| Mountain Dew® Kickstart™ Orange Citrus - Small | $1.00 | 90 Cal | 90.0 |
| Reduced Fat Sour Cream | $0.50 | 45 Cal | 90.0 |
| Nacho Cheese Doritos® Locos Tacos | $1.89 | 170 Cal | 89.9 |
| Power Menu Bowl | $5.69 | 470 Cal | 82.6 |
| Red Sauce | $0.25 | 20 Cal | 80.0 |
| Nacho Cheese Doritos® Locos Tacos Supreme® | $2.39 | 190 Cal | 79.5 |
| Power Menu Bowl - Veggie | $5.59 | 430 Cal | 76.9 |
| Black Beans | $1.49 | 50 Cal | 33.6 |