Demons and devils. They've been heavily neutered into what lowkey13 noted, big bags of hit points and multiattacks that lower level PCs can easily handle. Demon Lords from Out of the Abyss were chump change, as written, to level 14 mortals. I've done a few things: (1) give outerplanars back some of their 3rd edition spell-like abilities, legendary or bonus if an especially nasty being, and (2) got the Monster Talents homebrew material from DM Guild (which provides a way to beef up monsters with some of their old-school abilities without breaking the CR system).
For me, making monsters interesting is a combination of action economy (legendary, bonus, and reactions, doing more than one thing), use of the environment (taking advantage of terrain, including as a weapon), and dynamic options for foes (more than simply melee attack and done).
As to your werewolf, there's nothing wrong with you beefing them up if they seem too simple. Increase their hit points or strength. Make the lycanthropy save harder. Use more descriptive text. Given them a savage bonus (if they drop an opponent, bonus bite attack or the like, as if blood makes them more aggressive). Give them a howl feature, that it is so chilling that a low DC save might be necessary to avoid that knee-quaking fear, induced primally so that even experienced warriors might shake.