It's not so much what they value the most, but what they value the least. Early game, the AI tends to self-research Alphabet or Iron working, and no one bothers with Aesthetics. The trick is that you research Aesthetics yourself, and trade it around to everyone else, picking up Alphabet and Iron working as well as lots of little things you may have skipped like Archery and Sailing. The trade off is that Aesthetics has only marginal utility unless you're interested in its wonders, though it does give you an edge on Literature and the Great Library and the various epics.
Other good trade bait are the techs along the Liberalism line, as the AI does not prioritise techs when the one-off bonus has been used up. In particular, Philosphy (great scientist bulb this, the AI does not prioritise it when the religion has been founded), Education (because its bloody beaker expensive) and Liberalism are great trade-bait once the Liberalism race has been won. Later in the game, Biology is of low priority for the AI, even though it's incredibly useful.