While I do like CKII, my fondest memories are of CKI. I was once playing the count of Arborea (Half of Sardinia) and was gearing up for my inevitable showdown with Tuscany (Who always declares war on Arborea very early, with pretty much unstoppable forces) when an Irish count decided it wanted my lands. I showed him what-for, and sat my son there to rule while I fought a battle of life and death on my little island off the coast of Italy. I won. I even managed to take Corsica and the rest of Sardinia, tripling my size and power and making a nice little base of operations.
Then I died.
It didn't take long for me to notice I was now a king. Lo and behold, while I was off dicking around in Italy, my son took his ONE dirt poor Irish province, and conquered Ireland, becoming King of Ireland.
And Spain. He had half of northern Spain conquered by the time I took over. Without me noticing. At all.
I've never been so proud of a virtual son.