I don't think that
benedictio would be a very appropriate word for "blessing" here, to be honest. It means more the actual act of blessing, like the words spoken by a priest (the word
benedictio literally means "well saying/speaking", i.e. like you're wishing somebody well). It doesn't really mean "a good/desirable thing" in the sense that you're using here. A much more standard Classical Latin word for "blessing" in this sense would be
bonum (literally "a good thing") or
fortuna ("fortune").
You can also probably leave out "human" as it's pretty clear from the context that you're talking about a good thing for humans and not for, say, tortoises or angels or something like that.
If you really want the idea of "human" in there, you might use
hominibus = "for humans".
"Death is the greatest (human) blessing" =
Mors fortuna (humana) optima est/Mors fortuna optima hominibus est.
"Death is the sweetest (human) blessing" =
Mors fortuna (humana) dulcissima est/Mors fortuna dulcissima hominibus est.
"Our ultimate death is a blessing" (I suppose here by "ultimate" you mean something like "inevitable" and not "last", since generally a person can't die more than once?*) =
Fatalis mors nostra bonum est/Fatalis mors nobis bonum est. (Here again I would prefer the second option: "Inescapable death is a blessing to us".)
"Death is only the beginning" =
Modo incipio/principio nobis mors est. (Literally "death is only a beginning for us").
"Life is a waking dream, death is going home" =
Vita est somnium vigil, mors reditus domum. This one is a little bit more troublesome; I'm not sure that
vigil is quite the right word for "waking" in this sense, but I can't find a better one.
Anyway, others may have additional suggestions.
* Though in this time of modern medicine and the widespread resuscitation of those whose heartbeat/brain activity has ceased, that's rather more debatable, I suppose.