Okay, let's look at the standard mobs to see which is most dangerous:
Skeletons: Ranged attacks, difficult to avoid their attacks, can easily knock you off of ledges. Often hit their own buddies, resulting in infighting. Blocking can help reduce damage from arrows.
Zombies: Slow, melee only combatant. Can knock down wooden doors. Slightly more durable then skeletons and creepers. Avoiding their attacks is a better idea then blocking them.
Creepers: Exploding monsters that blow off a chunk of the landscape and part of your house. Shouldn't get a chance to explode if you know what you are doing when fighting it. Like to try and creep up on you. OCD inducing. Can be used to gather stone without the need for a wooden pickaxe. Hates cats, and will always try to stay a certain distance from one. Blocking makes a big difference when trying to survive their explosions.
Spiders: 2x2 mob that can climb walls (but not ceilings!). Easiest mob to keep off your property through certain construction practices. Likes to jump at you when attacking, making it harder to hit. Deflecting their jumping attacks works better then blocking them.
Endermen: OCD inducing mob that can move certain blocks around. Teleports to you and attacks you if you look at it. Teleports away if you hit it. Does not teleport away if you hit it in the legs. High health and high damage - an individual enderman can be more dangerous then even a group of mobs. Hates water. Likes to prematurely set off sand/gravel pitfall traps in custom maps. Trying to block their attacks only results in you getting hit again.
Looking at this list, I still say skeletons are more dangerous then creepers. Creepers can be avoided. Skeleton arrows can't.