Antifreeze. Apparently it tastes sweet so chugging a whole bottle probably wouldn't be too unpleasant. It is survivable though if someone intervenes.
Potassium chloride. You can buy it in supermarkets as "low Sodium" table salt. When dissolved in water and injected into a vein it causes a spike in Potassium levels that your heart really doesn't like. So it can cause a heart attack. If I recall correctly it is, or was, an ingredient in the lethal injection. Apparently people used to use it to murder people because for a period of time they didn't or couldn't test for it so it'd just look like the COD was a heart attack. They can tell now so no one uses it anymore for murder.
That being said, if you're depressed then don't kill yourself. Depressive episodes resolve eventually with or without treatment. Treatment tends to shorten them and reduces the severity but even without treatment they spontaneously resolve eventually so you just try not to kill yourself each day until youre out the other side. However if you've committed some atrocities and the cops are closing in on you or you're about to lose a war and be held accountable for your war crimes then either one of those will fit your criteria. The latter probably leaves less to chance assuming you used enough of it.
