Try Barcelona; Ramon Berenguer has no royal crown, but no one rules over him, and the weak crowns of Aragon and Navarre are within your graps just at the beginning. Usually, I marry one of my sons with the heiress either of Aragon or Navarra, while I conquer Lleida and Tarragona as fast as I can. Usually Albarracín, Calatayud or Zaragoza join the party too, in which case you still can handle it if you can summon troops from the counts of Urgell, Empúries and Roussillon.
I think Barcelona is the strongest option, and one of the funniest, to take over Iberia. The problem comes when Toledo, Badajoz and other powerful emirates declare themselves vassals of the Seljuks (a common error in this game). Then the Seljuks will come, no matter what's their situation, with an army of more than 80.000-100.000 men, to kick you ass with no solution.
You can still try to avoid this by keeping good relations with the Muslim territories while they are vassals of the Seljuks. Eventually, they will collapse and you will be waiting for it. Meanwhile, fight against your Christian brothers.
By the way, this game needs a way to recreate the weak Muslim emirates being "parias", dependant territories, of the Christian kingdoms, with the Muslims paying a tribute to keep the Christian warbands away.
Take care!