Soloing, like raiding, comes with two types, the casual and the hardcore. It really depends on which type you are and what kind of content are you comfortable with pursuing.
I myself have been dedicated to soloing for the past year or so with my shadow priest. Yes I could have used a DK for all of them, but really, using a DK just takes out the fun and challenge for most bosses. Everyone knows that DK (and hunters) are the top soloing classes. Priest is somewhere among the average pack. The challenge I have set myself over the past year or so, is to be able to use a priest to solo anything a DK that's 1 or 2 tiers ilvls lower can solo. For example, if my priest was T13 geared, I aim for bosses that a T11 or even T12 geared DK can manage to do.
Can't really say which boss is the hardest I've done because they all seemed equally as hard due to the fact that I always attempt them first time with bare minimum gear. However months later after I get a lot more geared, those bosses that seemed really hard suddenly becomes a piece of cake. At level 90, with just over 450 ilvl, I have only managed to do Halion, and it is most likely a world first Halion solo for priests at that. Due to my busy raiding schedule I have not soloed anything else. And yes I had help for the first 3 mini bosses and trash.
Bosses I have soloed as shadow during patch 5.0.4 at level85 includes:
Northrend Beasts 10N
Twin Valkyr 10H
Faction Champions 10H
All these bosses are also completely doable for a ilvl463 heroic 5man geared spriest. However I really doubt any disc/holy spec can pull off all of them.
I have also posted videos in these 2 topicshttp://us.battle.net/wow/en/forum/topic/4216984182http://us.battle.net/wow/en/forum/topic/3595573772