Oh dear, there are so many options and they all depend on how... Obvious you want the drama. I find something poetic about Santa laughing himself to death, personally. So that would probably be my pick.
However drowning and starvation tend be the most practical in normal play, requiring only proper walls or fencing. You can rationalize them pretty easily, too. (Swam too soon after he ate; someone accidentally locked him in the basement. You get the drift.) Fire is usually also listed here, but I don't tend to like risking my other Sims and the house. It certainly is dramatic, though, and as long as you are starting over a new life it can be a nice bang to end with.
If you are looking for something closer to fire but not quite as risky to everything else, electrocution can be good. Lightning is too unreliable to wait on normally (plus charged state and all that gives too many passes), but broken electronics can be good. I liked this method more in the older games, back when no Sim I loved touched anything that sparked. Now it is much more rare to lose them, but bad mood means carelessness, eventual death, and a natural explanation all in one!
And for the times you just want them dead this very instant, with Cottage Living you can click cheat death by rabbit for the now most convenient death so long as you do not mind cheating.
