5e here.

I'm with you on "monstrous races." 5e's spin on them is way to cool to waste on cannon fodder. Why send them out to slaughter a dozen orcs when I can make them babysit the Orcish ambassador?

My signature monster is "Bandits." I use the CR1/8 Bandit stat block so often that I should get a tattoo of it. It's just stupidly great as it is, you couldn't ask for a better "never the wrong answer" block. And that's before you start switching up equipment (which is actually huge in 5e) or adding racial bonuses (DMG page 282, "NPC Features.") If you get bored of human ones there's 25 more right there alone. You can basically use them for "any humanoid that hurts and/or threatens humanoids for a living" situation, including an angry mob. And I have.

My favorite way to use them, though, is part of a "bandit complex:" I have a single sheet of paper that has the CR1/8 Bandit, the CR2 Bandit Captain, the CR1/4 Riding Horse, and the CR1/8 Flying Snake on it. You can build a ludicrous amount of different encounters out of that, and I have.

If you need to drop in a caster, refluff the CR2 Orc Eye of Gruumsh for a support caster and the CR2 Cult Fanatic provides a great offensive caster. If you're starting to get to the point where there's just not enough challenge left in just <=CR2 without using more tokens than you want out at once you can throw in a refluffed CR4 Orc War Chief or 6 to keep the numbers on your side just a little longer. Honestly, you could almost run a whole "urban/piracy" game on just those blocks alone. And I have.