In terms of mobs, creepers. I don't know how many times I've died returning to my home in a ravine or cave when they fall on top of me. Wither Skeletons are also a pain sometimes, as well as cave spiders, blazes and zombie pigman. And the Wither, for obvious reasons
In terms of other things, a player on the server I play on that I used to be good friends with (until they teamed up with someone who griefed my city, then later when I got into a fight with another griefer of the ethics of griefing, teamed up with a good friend of mine and blew my new base to crap) just obtained a [LEGENDARY] weapon named the [Fang of Gahzrim], which is a Diamond Sword (Sharpness VI, Looting I, Fire Aspect I, Knockback II). Works out great for me
Myself? I tend to die via suicide. Even after 3 years of playing, keepInventory is always on XD
But otherwise, it is fall damage. Since 1.6, mobs become harder the longer you stay in an area. Creepers now instantly explode, launching me off my platform...
Rollback Post to RevisionRollBack
Watch this! I can make your read this text! See? MAGIC! You have to use your jetpack! WOOSH!
its rare for me to die from one singular mob. i usually die due to a..mob...of mobs. most recent example: took on a group of mobs, killed most of them fine. came down to a witch, hit me with poison, it wears me down to half a heart, i'm running away from that one last mob and....shot in the back by a skeleton. i guess skeletons are usually the ones to finish me off.
I would have to say mobs are the number 1 reason for my deaths in minecraft. To be specific, it's the skeletons. Before 1.5 was release, zombies were to blame for most of my blames but since 1.5, skeletons have killed me way more often. Of course the reason is because within the 1.5 update, skeletons were really buffed. Being able to shoot from 16 blocks away (previously 10 blocks away) as well as their shooting speed increasing as players get closer.
I find on hard, zombies can get the upper hand on me fairly quick when I'm not paying attention. It's mostly since a lot of the time I'm not wearing armor. If I gave half a crap to go in the Nether, it'd be pigmen. They attack me unprovoked a lot of the time.
Since I usually play on peaceful mode, I don't die very often.
However, when I go out monster hunting on easy mode, I occasionally do die. It's almost always a skeleton.
I try to fight an enderman now and then and always lose. (Really, I haven't successfully killed one yet.) However, endermen don't attack unless you provoke them. Skeletons will sometimes attack before I've even seen them.
Even using my shield (from Mine&Blade), I can't block all of their arrows. Approaching them without taking lots of damage is almost impossible for me. If I have a bow, I can generally kill them without too much trouble, though.
Not much else generally manages to kill me. I'm careful enough when digging that falling sand and gravel don't generally kill me and I fall into lava very rarely. I also generally don't fall to my death. I have had some nasty falls, but I usually manage to steer myself into water or the fall just isn't fatal.
Mobs without projectiles can't usually kill me. (Blazes have killed me and I will probably get killed by a ghast at some point as well.) There are a few exceptions. When fighting through a "battle tower" (from the mod called "Battle Towers"), I almost always die at least once or twice. Also, the werewolves from "Mo' Creatures" always, always manage to kill me. (Those werewolves are very overpowered, though, so that isn't much of a surprise.) Basically, either there need to be lots of hostile mobs or the mob needs to be very powerful.
PVP isn't an issue for me since I only play single player.
I haven't drowned yet, either. I have had drowning damage, but it's never been fatal yet, even when I was lost under ice.
I did once die from falling into the void. What happened was that I tried to use the teleport command (yes, a cheat) and got the Y and Z coordinates mixed up. I tried to teleport to the center of the map (which was my default before I started looking at waypoint coordinates or using the "teleport to waypoint" function in Zan's Minimap) and wound up in the void. That was embarrassing. It's only happened to me once, though.
tl;dr - Skeletons that I encounter while hunting monsters on easy mode kill me more than anything.
Rollback Post to RevisionRollBack
The source of my intention isn't really crime prevention; my intention is prevention of the lie! Yeah, welcome to the Scatman's world!
Falling has become a close second for me, at least back in 1.2.5 when I was playing with the mods from Direwolf20's Season4 LP. Two of those falling deaths were in the Eldritch Monoliths of Thaumcraft2 after I had opened one and tried placing ladders while sneaking. The other was with a Portal Gun from iChun's mod. I shot it at the Moon then the ground, got sucked in and fell from the map's maximum height.
I did come close to dying in lava recently - got down to 1 heart before I reached a nearby water pool. This was from a lava lake on the surface. I was collecting it in cans for my machines in the FTB Direwolf20 modpack for MC 1.4.7 and jumped the wrong way.
Creepers kill me most because they sneaky and blow behind my back and then skeletons, mainly if i dont have a bow... they just suck because to kill them with melee you will have to take damage somehow and sometimes some damage taken can make a difference.
I'm just going to call it the Nether that kills me. Mostly blazes, but sometimes other mobs. All of my deaths recently have been from lava, which I consider to be sent by the Nether to kill me in the Overworld.