Mehrunes Razor is a good dagger to get ahold of if you need a second one. Best dagger out there though is the blade of woe. It's possible to get 2 blades of woe if you have at least 70 pickpocket and the perk to steal your enemies weapons when you start the dark brotherhood questline. As far as daggers go, all else that I can suggest is getting one with soul trap.
If you're into swords, I suggest hitting the thieves guild quest line at level 46 like Dagmar said above. At 46+ both the Nightingale sword, and Chillrend are at their highest strength, and Chillrend actually has the highest base damage of any sword in the game if gotten at level 46+ as well as having an awesome enchantment(and looking cool).
As for bows, Finriel's end is a good start, but the cream of the crop is the Nightingale bow at level 46+. I believe that at that level it is the strongest base damage for any bow in the game.
No assassin is complete without armor right? Well, the thieves guild armor will provide better armor rating, but the shrouded/ancient shrouded armor has better enchants. Predator's grace is an alternative for the boots, and if you aren't going to be using a bow very often I would suggest taking the shrouded hood rather than cowl for the sneak bonus over the archery bonus. Also, you may think about the Jester's clothes/Cicero's clothes. They don't provide any armor rating but the bonuses are pretty good. Boots and gloves of the old gods are great for a sneak archer and finally I must mention my favorite armor set: Linwe's armor. It's inferior to the shrouded armor but it does provide decent bonuses and armor rating, it also looks awesome!