Are you finding Skyrim too easy for you? or just getting an opinion from members what's hardest? not sure what your asking. Personally, I think in order to make the game the hardest is to set rules and roleplay. That's something that is not on the menu to adjust. There's toooons of sugestions on rules/roleplayin in order to 'hardin' up ANY dungeon or quest. Try not drinking potions or restoration spells....that should increase the difficult a little. lol or go in naked barehanded!! ha!!
lvl 43 will net you fully leveled items for the Thieves guild... Try becoming jarl of all holds too... Can't think of much really, get bounties for dragons and work with the blades perhaps, or try adding interesting techniques to your character, start using magicka more, or battle more, or sneak, etc.
Edit: Go werewolf frenzy or lvl 4 vampire, complete all Daedric quests, lol