Illmatic is just perfection to me. That's the definition of a rap album that people are still trying to figure out. Nas doesn't slip up once on that record, and the weakest track on it ("One Time 4 Your Mind") would be a huge club banger for someone with lesser talent.
It Was Written is still a good/great album with me, but it just reeks of Nas trying to overdo the crossover thing. It felt inorganic like he had to do these tracks for the sole purpose of being recognized. I remember reading somewhere that Illmatic was really only appreciated here in New York City, and it took a while for everyone else to catch on because it was so underground and unpolished. Also, it said that had Nas continued on that Illmatic-type path (his second album was originally going to have Marley Marl of all people doing production), he would be in the same boat as someone like Immortal Technique. The under-performance of Illmatic and Biggie's meteoric rise to fame got to Nas and that's why he started overcompensating for it. Plus, some songs either start running together ("Suspect," "Nas is Coming") and are bland, or are just awful ("Black Girl Lost," one of the very few Nas songs I cannot listen to). Then again, Nas is still rhyming for his life and doesn't compromise any of his talent. The Trackmasters did great production work, and at least three songs are indisputable classics ("The Message," "I Gave You Power," "Take It in Blood"). Still, Illmatic takes it for me. A great comparison would be It Was Written to Reasonable Doubt or even some of Nas' later work (I Am..., Stillmatic).