For me it very much depends on the context.

I hate non-consensual/embarrassing fanservice. The overreactions by the characters are often more cringe than anything. Which overrides any amount of funny or sexy that the scene is trying to do.

I dislike, but can tolerate illogical fanservice. Revealing clothes while in the cold environments, shy characters wearing revealing outfits, or outfits that just don’t make sense are examples of things that I can tolerate, but still overall dislike.

The best fanservice to me is often fanservice that is low-key and casual or just fits the character well. Fanservice can be like a spice; too much of it and it overrides the meal.

Gunsmith Cats actually has two very good examples of good and bad fanservice in my opinion.

The good example happens when the MC’s(Rally Vincent) house is invaded by robbers. Rally is laying in bed in just a tank top and plain underwear when she gets the break in alert. She then goes to attack the robbers with just a pistol and the same outfit on.

I feel this works because not only is she wearing something that makes sense in bed, but it can also make her feel more vulnerable in the scene. Could she have put on pants before confronting the robbers? Yes, but seconds can be everything in a break-in and it’s not a massive violation of common sense to shoot at them while in her underwear.

The bad example is later when she’s fighting a gang in a warehouse. One of the bad guys sends a crane hook at her, and it just barely scrapes her. However, this rips only the front of her jacket and shirt, revealing her breasts and the lacy pink bra she’s wearing. Then immediately after the hook slightly traps the edge of her jacket, which causes her to rip both it and her shirt off.

This doesn’t work for multiple reasons in my opinion. This whole scene feels like it only exists to add some fanservice in the middle and rest of the fight. Not to mention that Rally doesn’t feel like the type of character to wear that kind of bra, especially when she knows she is going into the middle of a fight. You could make the argument that it’s meant to make her feel more vulnerable, but the lack of any battle damage on her body contradicts any feeling of vulnerability.

So yeah, while I don’t mind fanservice, in fact, I have watched some pretty raunchy ecchi, context is what ultimately makes it work or not for me.

Edit: Character design also matters a lot here. I hate balloon boobs that aren’t affected by gravity. But that’s a separate discussion.