In my second playthrough I have leveled smithing, enchanting, speech, sneak and archery to 100%. Next I decided I wanted to level up my block. I had a glass shield with 25% block all damage enchantment on it, I leveled up in probably 10 to 20 minutes. I have to say this was the fastest skill to level. I would find 3 or 4 enemies and back myself into a corner and let them continually hit me. I was leveling up so fast and not taking any health damage at all! Leading me to the topic question, what skill do you think is the fastest and easiest to level up?

- I would say BLOCK.

the same principle applies to light and heavy armour - letting a lower level enemy attack you raises that pretty quickly.

I also find pickpocketing really fast to level.

Pickpocket.

PIckpocket.

Enchanting levels damn quickly, too.

Fastest skill for me was sneaking. I am always sneaking and it was the skill, which reached 100 before any other. You can just use the text console to set a skill to 100 and that you do not have to stand in a corner like you did. Know that when you explore all caves and ruins (>300 locations) with one character will you spend enough time to get more than two third of all your skills to 100 without doing anything for them but to use them occasionally. There is no need for any skill grinding if you only have enough patience. The worst that you can do to is that you have your skills at 100 and find that there is still so much to do in the game, but cannot train any skills up.

PIckpocketing is probably the fastest with Sneaking, Blocking, and Smithing right up there, too.

I have never picked a pocket...

You can always take people's gold without having to worry about stolen items. The game does not know about stolen gold.

Some NPCs carry journals or notes with them. You really should "look into it".

At one point my gold was flagged as "Stolen", but after I put it in a container and removed it cleared it. Ever since then I haven't stolen any gold...not even from detestable people like the Black-Briars.

Alteration : Detect Life & Telekinesis
Find and enchant to reduce mana need for them. Then stand in some marketplace.

Pickpocketing. When I decided to make a thief which relies her skills solely on her own practice (no training), I was amazed by how fast it levels. I pickpocketed the whole of Whiterun and Riften in less than two hours and got it to 100.

Don't think sneak level fast, I almost always sneak then attack or in dungeons and it's still not 100 but 90 something at level 49.
Yes it probably level fast if auto walking into wall with multiple npc on other side.
Pickpocket level fast same with smithing,

Speech, Smithing, and Sneak.

Speech has a dialog loop you can exploit in the Riften Meadery. (Ask the seller about Maven). I think it requires ~20 speech or so, but you can also bribe him for the information and bump it up a few points if you need to.

Smithing can be leveled via iron daggers, I tend to craft around 100 at a time and sell the daggers to a blacksmith. I can get it to 100 with around 5,000 gold, give or take depending on prices.

Sneak can be powerleveled if you followed Hadvar out of the intro sequence. Witness the general scene with Alvor, and when everyone is out and around, backstab him constantly. Be sure to stop when he turns around.
I also find it best to set the game on master so you don't KO him all the time.

Even without that exploit, sneak can level pretty quickly if you just successfully sneak attack everything you come across. Draugr are really good for this. Hit them while they sleep.

Pickpocket

Sneak, smithing, enchanting, block, and depending on what type of armor you're focusing on Light or Heavy armor but only if you fight lots of giants or use a sheild a lot.

Illusion by spamming muffle. Do the Illusion master's quest for temporary fast regen and dual cast muffle for 20 minutes.

conjuration spam soul trap on a dead body or spam bound swords on a rock next to a enemy

My first characters in the game I cheated like crazy with console codes. This time I have played legit and is much more enjoyable.