Hmm. So far I've had the most success as Karthus top lane using the following build:
-Start Chalice+Boots+pots, taking 1 point in each skill.
-Rush Hextech Sweeper and Sorceror's Shoes-the former for the utility/CDR and the latter for power.
-Build Giant's Belt-> Rylai's Crystal Scepter.
Rylai's and Sweeper are your core items.
After this, you need Grail, Void Staff, and one defensive armor item that has Chain Vest in it. Options to consider are Frozen Heart, Zhonya's, or Warden's->Randuins. (If you take any armor item that isn't Frozen Heart, replace your Sorcs with Ionians.) You end up with lots of damage, bulk, AND utility.
Initially, try to keep your distance-use your E when the enemy gets too close, but abuse your Q poke whenever possible. Your W is your team's best friend for boosting your team's DPS-it also works decently with champions who like the slow (like Shyvana.) Use your ult for DPS or kills. R->Q->W=E, leveling E when you need push and W when you need to team support.
Revive+Garrison are my usual summoners. I go 21-1-8 with movement speed over mana regen.
You can eventually swap out bot if the need arises-once Karth gets his pushing power...
WotA, I've found, feels like a noob trap on top laner Karthus. Rabadon's has the worst build path in the game and takes up an item slot I could easily spend on Void Staff.