GIMP Version: 2.8.4
Operating System: Windows
GIMP Experience: Basic Level

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Hello, I have few questions about sharpening:

1. In your opinion, what is the best sharpening tool in GIMP? I usually use unsharp mask, and after I found wavelet sharpening, I use it too.

2. To enhance a picture, sometimes I do a combination of noise removal (smoothing) and sharpening. Which one should I do last? You know, because those two steps actually "cancel each other".

Maybe my questions are asking too much, but you guys are GIMP gurus, so maybe you know better than me

Thank you for your help

It truly depends on the image. I wish I had a better answer for you.

You go with what looks right. As in, if you use non-destructive methods, you can always just delete a layer that went too far. You can blend 2 layers together if the effect was more than you wanted etc.

As far as my favorite method to sharpen an image goes, it's a small trick I picked up around here some time back. You make a new from visible copy, gaussian blur it by 1 or 2, set the layer mode to grain extract, new from visible this mostly gray field, turn off the blurred layer, grain merge the greyed layer and use layer opacity to adjust it. You can blur the gaussian layer more for stronger effect, and tweak opacity to lessen it before merging all that together. It's more hands on and low-tech, but it gives the subtle touch that these images need sometimes.

Finally, I would do any smoothing and despeckling etc before your final sharpen. Again though, depends what your image looks like and what feels right when you look at your different results. Go either way with it and preserve layers of both directions and keep what looks cleanest.

For standard photo retouch i first denoise using GMIC> enhance> smooth (anisotropic) taking care to not blur the edges but only smear the noise.
After i sharpen, i don't like at all Gimp unsharpmask,
i use instead Unsharpmask2 (that has a exact equivalent in GMIC>Details>sharpen(unsharp) moving "light sharpen" to 0 or close and using only dark sharpening , that has a more natural looking (and if overdone has a more interesting graphic effect then standard unsharpmask )

If you do the contrary you sharpen and enhance all the noise , then you are forced to use more dramatic setting to smear the noise,
this may create interesting special effects BUT is no good for standard editing

And btw is not true that they just "cancel each other" if carefully used good denoising filters didn't blur, at least they don't blur the edges neither the details but only the noise

Even a no sophisticated filter as gaussian blur may be used to blur just the noise, i do always with 3d renders :
on a large image let say 4800x3600 a gaussian blur of 1 has no any visible blurring effect,
EXCEPT for the typical , very little and sharp dots of white or black noise
... a noise that may fool more "smart " denoise filter because so sharp and contrasted that may be read as "details to preserve"
(it will not fool wavelet denoise, but many of the others that identify "the details to preserve" mostly from the contrast with close by pixels )

So first remove at least the most tiny and sharp noise, then sharpen

and yes , after sharpen are few cases where may be useful denoise once again ,(because as may reveal hidden details sharpening may also reveal noise ) but in most cases is not needed