I'd rank the bosses in this order:

1) Void Reaver
2) Lurker
3) Fathom-Lord Karathress
4) Morogrim Tidewalker
5) Hydross The Unstable
6) Leotheras The Blind
7) Al'ar
8) Vashj
9) Solarian
10) Kael'Thas

I wanted to put Al'ar lower in the ranking, but most of the SSC stuff is so damn easy nowadays so it's safe to assume to just do Void Reaver once per week and then focus on SSC. You can skip Al'ar for Vashj too, really. What's the use of focusing on two instances anyway :P.

You find Solarian harder than Vashj? Vashj has tons of things going on with lots of coordination and communication required. Solarian is a pretty simple fight that pretty much only requires quick healers. Good DPS is a solid bonus too.

The TK bosses aren't any harder or easier than the SSC ones. If anything I think they're easier. I don't believe order matters too much except "every boss < Vashj < Kael'thas." If pressed to give an order for progression, taking into account difficulty and loot, I'd say:

1) Void Reaver
2) Hydross (over-hyped, not that hard, and not much trash clearing)
3) Lurker
4) Leotheras (Morogrim is more frustrating, feels more luck based when you get huge MT spikes or 3 healers graved)
5) Karathress
6) Morogrim Tidewalker
7) Solarian
8) Al'ar
9) Vashj
10) Kael'thas

But again, I don't think order matters too much because they're all at roughly the same level until you get to Vashj and Kael'thas.

I honestly don't agree that Solarian is harder than Vashj...
After you can kill Morogrim, I'd make clearing of Al'ar's trash more productive and kill both VR and Solarian in the same run...

Nah, I misplaced Solarian. Should be after Al'ar, I guess.

Solarian isn't harder than vashj - but one could see doing Vashj first - considering what a royal pain in the ass solarian is. However, for our guild we spent more time killing solarian pre 2.1 than killing vashj... go figure. Of course, I didn't consider pre-2.1 vashj really on the table for kills so we never bothered.

Anyway, you guilds working on ssc/tk25 keep at it - the world is 100 times better once you're done with those buggy zones (although hyjal is still the most draining zone in existence, so yeah, black temple is good).

I'd agree, SSC and TK are worlds from where they started at, and are a lot more enjoyable because of it, but once you move on from them it just gets better.

For some reason I actually really like Hyjal too, even though the second boss seems the hardest in the zone (although Archimonde does require people to time the tears well, which some people have difficulty with - even mages when they can slowfall, go figure). Maybe it's just assist training Gargoyles with the Taurens beating on them, but theres something enjoyable and different about it.