Fire Aspect, Sharpness, and Looting are my favorite. Knockback is awful.
I wonder how you would handle this (just ordinary zombie spawns trying to invade a village):
That is actually a fairly regular experience as this shows:
(while caving, often due to how I made zombie spawners spawn mobs every 5-10 seconds (instead of 10-40 in vanilla) with increased spawn counts and caps, plus in 1.6.4 the tracking range is more like double the intended range so pretty much every zombie in the mob cap tracks you down).
Also, perhaps another way to judge how effective your sword is is to see how much damage you have dealt and taken:
In this case, I've dealt over a dozen times the damage I've taken with an average of slightly more than 2 damage, one heart, of damage taken per mob killed (note that this is on Normal difficulty but it can be scaled, wearing armor has no effect on "damage taken"; which is before any armor calculations, similarly, I've dealt an average of 25.8 damage per mob, as the game counts overkill damage (Sharpness V diamond = 14.3 damage per hit, 28.6 for most mobs); also, a lot of the damage I get is from poisoning, as cave spiders (which spawn true to their name, not just "abandoned mineshaft spiders") are often hard to hit before they hit you, almost like they have a larger reach than you do); witches (no difficulty scaling) have knocked me down to half a heart. Note also that I consider these stats to be inaccurate if you've used mob traps to kill many without getting hit by them, or killing them with a punch (only one damage dealt per kill); almost all of the mobs seen, aside from a handful of passive mobs (probably less than 1% of the total, now only the occasional bat I hit while mining), were killed in ordinary combat.
imo sharpness IV, unbreaking III, looting III. and then fire aspect II and knockback II are good additions but i personally dont care for knockback. also if you are early game and are using the looting enchantment to farm ender pearls for the end, i would recommend not using fire aspect as it makes endermen teleport around and they often die in random places far away making the pearls hard to retrieve.
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Personally I like Sharpness III, Looting III, Unbreaking III, Knockback I, and mending. Knockback over level 2 seems excessive and Fire aspect gets me set afire as often as not. I only use fire enchants on a bow.