There are two ways to define difficulty. One is in building, one is in play.
Building:
All classes have to pick utility powers and feats.
Both fighter builds have to pick stances, most of which have a very clear use, and a weapon to specialize in, which again has a pretty straight forward effect. That is arguably the easiest.
The cleric, has to choose a build, which in turn determines all it's encounters, and then it has to pick daily powers. It's pretty straight forward, but requires more choices than the fighter (and because they can technically pick from all cleric powers for their dailies, it could be made more complex quite easily, while many other classes only have utility powers they can access out of book).
The warlock is in the same position as the cleric, their build choice sets their encounter powers in place, and then it's a matter of choosing their dailies, which puts them in the same boat as the cleric.
The paladin as well is in that group, basing it's encounter selection on the build, leaving daily powers as the other choice. within that group I'd say the leader is the toughest of the three, followed by the defender and then the striker, as the more other characters are reliant on you, the tougher your choices become.
The ranger has to pick stances, a choice of weapon, and knacks. The knacks and/or stances come up much more often than daily powers do, so proper choices of those is a bit more key.
Similarly, the druid has to choose their build (which is a combination of high damage/low accuracy or low damage/high accuracy with both their own weapon and their animal companian), but also an at-will power, knacks and daily powers. They don't get multiple at-will options in play like the martial characters, so the choice of at-will initially is more important. Similarly, their choice of animal companion has an impact on their allies, as do knacks, and they have dailies on top of that.
The rogue has to pick it's tricks. While this is really the only option the rogue has to have (other than skills, feats, utilities like everyone else does), evaluating the benefit of the tricks is a bit harder than most of the other options. They are similar to the monk's full disciplines as they provide both movement and combat bonuses, and have some implied benefits (ability to get in and out of melee/flanking, etc) that make them a bit harder to evaluate than simple power choices like extra damage vs. dazing, etc. I'd say probably on par with the druid, maybe the druid is a bit harder though.
The top of the heap is definitely the wizard, as it has to pick a build, but still has to choose it's own encounter powers, in addition to dailies. On top of that, all power choices are doubled (outside of at-wills) because of their spellbook, which requires more than just pick the best (or the one associated with your build) power at each level ... you have to factor in how to best make use of the versatility. If you include that all existing wizard powers are technically available, the complexity goes through the roof.
In play:
The slayer fighter is the easiest. Set your stance, and apply your encounter damage bonus when applicable.
The scout ranger has a bit more difficulty in picking the best way stance, but otherwise they are more or less at the same level as the fighter, with the same encounter mechanic even.
The knight fighter does have to deal with the additional burden of the defender mechanics, but as it is an opportunity action and a static aura, there is no burden of "do I save this for later" involved outside of the same encounter power damage boost.
The warlock has the addition of dailies and encounter powers and when to save them or use them, but otherwise is pretty straight forward. Their attack options for at-will are simple, they have one melee and one ranged, so it's pretty much simple if they don't want to spend their limited resources.
The paladin also has dailies and a encounter powers like the warlock, but with a choice of at-will attacks that need to be picked from each time they attack, plus the defender mechanic makes for extra decisions needing to be made.
The rogue is constantly looking for combat advantage, which means making best use of it's tricks at each opportunity. It's enounter power however isn't much of a difficult choice, as it's a matter of either deciding to deal extra damage or turning a miss into a hit (which is doing a LOT of extra damage).
The hunter ranger has decisions similar to the scout ranger, but the two big differences are that you have the extra at-will options (you basically have 5 at-will powers which are modified by your stances) and your encounter is proactive instead of reactive (i.e. you don't decide to add the damage after you hit, you have to decide to try and hit with it, and risk 'wasting' it with a miss). Also, in general, as a controller your decisions are a bit harder, especially as you have many more viable targets as a ranged character than a melee character would.
The cleric has to manage it's resources, deciding when to use it's encounter powers, when to use it's dailies, plus when to use it's healing. And with unique encounter powers at each level, it's a tougher choice of when to use each than with the characters that have the same encounter power multiple times.
The druid has a harder time than the cleric. While it only has one recurring encounter power, it has to coordinate the movement of both itself and it's animal companion and how to best position that companion.
And of course, the wizard is at the top, with all the encounter/daily usage options, the ranged issue of tons of viable targets, and the daily choice of which powers to prepare.
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I left out the assassin ... I'd probably say just shy of the wizard (in play) ... the ability to only use the encounter once makes it a tougher decision of when to use it, you have 4 at-wills to choose from (or however many ... the most recent version isn't the complete version either) and you have dailies which, like the wizard, require you to not only pick when to use it, but which to prepare each day.
As for building, it's a bit easier. Your build choice gives you most of your at-will options (or limits them enough where it's a choice of 2 of 4) so the only other option is your dailies, albeit with more options than you can use at any give time. Still, I'd put that somewhere in the cleric/warlock/paladin range of difficulty to build.
