I didn't pick any in the poll. Each class has a hard time in specific cases, and each class has an easy time in specific cases.
I've probably had the most deaths with Rune-keeper, but only because I skipped an entire level's worth of quests back in the late teens/early 20's which put everything from 20 - 35 at the extreme peak of the difficulty curve. But even then, I didn't have much problems with anything.
And at the same time, I'd have to say Rune-keeper is the easiest because they're so darned good at both healing and killing.
Got an escort quest? have the npc do all the work while you sit back, paint your nails and throw the occassional heal.
About to die? Stun em, switch to healing and laugh with a full power and health bar as you start the zapping again.
I've heard complaints about Lore-Masters being difficult to level. And I really don't see it. ANYMORE. There were lots of issues with them in the early days of LOTRO, but as far as I can see, they're all fixed now. Especially in the late levels, Cynderwen is nigh unstoppable.
I've heard that Minstrels were having a hard time in Moria. Haven't seen it. Now... I did have to adapt my playstyle on Wilim, but he's an efficient killing machine now. Usually ends up with a full morale bar at the end of each fight.
Burglars? OMG! Everything's a cake-walk. Easiest time getting from 1 to 50 I've EVER had.
Captain I had a problem with simply because I really didn't like the class. (Might have had everything to do with Wyncen being an explorer and my 7th level 50).
Until overpower came into being, I would have said Guardian was difficult. But with overpower... CAKE
Hunters & Champions are overpowered.
And Gildir to 35 pre-Month of the LM! That was an accomplishment.. there should be a title for that! Really! My first character in Lotro was a LM. got to 20... deleted.. tried again... got to 22 and deleted her to not be seen again until cynder.
Bottom line: I've soloed each class up to 50 pre-moria and haven't thought any of them particular "hard". Each had to put off certain quests until a later level because of their strengths/weaknesses, but each have done marvelously.
And note: I'm very "aggressive" when solo-ing. I'm in orange mob territory 90% of the time and occassionally taking on reds.