What skill do you lose the quickest if you don't practice regularly?

Language, and writing.

If you speak a language and never use it, it will begin to rust. It will deteriorate, lose its luster and elasticity, become stuck, stiff, dry. Disuse will shrink something that once glistened and rolled into an impaired, clumsy, creaky version of itself.

Something similar happens with writing. If you write every day your writing gets better, exactly like legs when every morning you run up a long flight of stairs. If you stop writing, your sentences, once luminous, polished, almost musical, become dull and lifeless, devoid of poetry.

Both skills can be recovered if you go find them, pick them up and dust them off; if you take the time to lubricate and restore all your words and the way you want to string them together. Language is a sacred, trusty instrument, and if you give it even a bit of attention it recovers, and will one day again feel as if it was new.