I've mostly beaten the game once, and now I bought Hearthfire and DG and started over. I'm on PC so no DB yet. I decided to try to rake up all the achievements I didn't get in my first playthrough, either by laziness or because of poor choices. I'm trying to get the oblivion walker one, where I have get 15 daedric artifacts, and I'm finding it very difficult, mainly because it's soooo long, and it requires you to do some really bad things, things I don't particularly like doing even in an RPG... Fact is, I got bored after my first 8 artifacts, and started this topic to see if other people are trying to get all the achievements (getting them all is fun IMO), and which one is/was the most diffucult/hated for them! I haven't started a poll because there are too many!

The hardest is the Hail Sithis one from the Dark Brotherhood, since it's bugged for me. It isn't possible for me to achieve it, it doesn't registrer.

I had a similar thing happen to me. I got "With friends like these", steam registered it, then a couple of days later I opened steam and it got unregistered...

The achievement for killing a legendary dragon, just because you need to be an insanely high level to get them to spawn.

A few, mainly the ones I haven't done yet:

-Vampire Mastered and Werewolf Mastered: I simply don't enjoy using the forms (or being a vampire.)
-Legend: The only characters I've ever gotten above 50 have been either Oghma-glitched or deliberately set up as game mechanics test environments; so I've never seen a Legendary Dragon in-game. Also, I tend to not join the Blades.

And those that I have done... "Have" done... but wouldn't do normally:
-TG, DB, and marriage: These were obtained through playing around with the PCC. In this case, it was caqs. I've joined the TG... once. That character wound up not existing a few hours later.

Really, though, the fact that I used caqs should tell you that I'm not too interested in gaining the achievements. ^^