Interesting and challenging question
Here is my personal list. Not to mention that I also have a tendancy to change favorites often sinc I listen to a lot of flamenco (almost two hours everyday, thanks to long time commuting) :
* La caña (and the Polo) : I have heard two cañas in my yet short flamenco life. One from Antonio Mairena (the best cantaor for the jondo repertoire imo) and another performed by Pepe Marchena. Very emotional, intense and extremely complex in matters of compas and above all singing technique. Polo is also very intense but doesn't have the lament part that makes the subtle charm of Caña.
* Soleares : one of the palos I cherish most, especially the Alcala style which is the best ever according to me. I have heard singers like Juan Talega singing that kind of Solea and, man, it's amazing! Soleares de Frijones are also very interesting.
* Petenera : my favorite one is a Petenera performed by Fosforito (do you consider him old school or modern, btw?) accompanied by Paco de Lucia. Profound letras and high level singing skills required for this palo which means that the cantaor(a) will usually show his or her "guts".
* Tientos : a 4 beat palo but the letras are so beautiful. I admire the accompaniment which, when well performed, gives the impression that it is played 3 beats.
* Solea por Buleria : love the toque of old school players for this palo, like Sabicas (early period) or Ricardo. The letra are also very intense.
* Martinete, Debla y Tona : these palos are extremely deep and the letras often very deeply rooted in Andaluzian culture which makes them even more valuable. Mairena is my favorite performer up to now for Martinetes.
I should also add to the list Seguiriyas, Tangos, Buleria (especially the one from Moron and Utrera), Serrana, some Fandangos (depends on who sings and wo plays guitar), la Saeta, and the Garrotin (Sabicas). And still so many things to discover!!! I can't imagine that flamenco is that huge
