Most games have a slew of options available for monsters, bosses, and other combat opponents for player characters to come across and fight. I'm honestly not sure I can think of a game with combat as part of the game that doesn't have some form of pre-prepared stat blocks to handle this.
At the same time, it is almost a time honored tradition for GMs to make custom enemies for their games that are more than just renaming or refluffing an existing stat block. Custom stats, custom attacks, custom powers, custom defenses. Do it right and you get a unique encounter that can add to your game. Do it wrong and you get something that is too easy or too powerful that doesn't feel as good.
My question is, do you do custom stat blocks? If so, how far do you go? Do you find it worth the time investment to do? Does any particular thing stand out as having worked particularly well?
With D&D 5e I mostly stick to pre-prepared stat blocks, but can be very liberal about just extracting the mechanics and reskinning all the other stuff around it. Sure, sure, the stat block says "Beholder Zombie" but it is actually a Techno-magical Necromantic construct housing a prism in a cracked rib cage that shoots off random rays, not a beholder. Yes, I know the stat block says it is a Hobgoblin Warlord, but it is actually a Dwarven Lord leading his elite bodyguard unit.
I've started to dabble in other customizations, and even making entire stat blocks, but in general I find that the amount of work to the amount of game time isn't always worth it. For some encounters I want it to be special enough I'll do it - for example, I'm making 4 custom Death Knights for one of the campaigns right now - but by and large I find reskinning and tweaking does a better job of adding enough customization to feel unique without costing me hours of research, numbers, and planning for something that will be lucky to last 5 rounds.
I did much the same with Fantasy Flight's Edge of the Empire.
However, when I ran L5R 3rd ed and 4th ed, I did the opposite. I almost never used pre-prepared stat blocks and instead made custom enemies. The big thing here was that I knew the system well enough, and the system was supportive of this, that I could whip up a stat block in minutes. Most things only needed their attack roll, damage roll, wounds, and TNtbH. if they needed more they just needed values for their 5 rings.
How about you?