There are many reasons why a mod might not have made the cut, including but not limited to: I tested many, many more mods than made this guide.
I've chosen a variety of mods to suit playstyles and Classic philosophies, from bare-minimum mods to feature-rich addons that you can tweak to your own exact specifications. This is my opinion about what I found to be the best addons for helping you level your characters in Classic, whether it's your first character or part of a stable of alts. While 'Best' is a subjective term, in this guide, I will dig through the best addons for leveling in Classic and indicate ones I consider essential.