I've been wondering about this for a while: in Orthros, there is a prayer which names a long list of saints, such as St. John the Forerunner, St. John Chrysostom, a number of martrys and so on. All saints are good, of course, but I was wondering why these particular ones were chosen. Thank you.
The list has evolved over time, and has minor differences from place to place. Basically, the saints remembered represent some of the most frequently supplicated to intercede for us to God. There is a predictable order to the prayer:
1. We're asking God for His mercy on all of us (and the prayer is specifically addressed to the Father - see 16 below)
Through the prayers of:
2. The Theotokos
3. By the power of the Cross
4. Protection of the bodiless powers a.k.a. the orders and hosts of Angels
5. The Prophet and Forerunner
6. The Apostles
7. The Hierarchs and Teachers of the faith.
8. The Martyrs
9. Our God-bearing Fathers (which, for some, means the ascetics, and for others is more broad)
9a. (Optional) The Ascetics
10. (Optional) The Unmercenaries
11. (Optional) Saints of local importance (even though the should be remembered in their proper place above).
12. Saint of the Church (if they're not covered already in #2-5 above)
13. Theotokos' parents, Joachim and Anna
14. Saint of the day (if the celebration of that Saint is of particular importance*)
15. "And of all the Saints"
After the 12x "Lord, have mercy," the prayer is completed by:
16. Invoking the Grace, compassion and Love of "Your only-begotten Son" - i.e. Christ - and the Spirit.
Within each category above from #6 onward names of saints can be added. Yes, we have a "standard set" that seem to be remembered everywhere - but that is because of their impact on popular piety, or the overall life of the Church: The 3 Hierarchs, the Alexandrian trio, Sts. Nicholas and Spirydon; Sts. George, Demetrios, the Theodores, and Minas.
However, in various places, others are added. One hieromonk who served at the Theological school would always add ascetic saints. Fr. Seraphim Dedes, to his Sunday Matins pakcets, has added the female martyrs. In the Greek practice at the end of the hierarchs list we add St. Nektarios, and after him we add the hieromartyrs Eleftherios, Charalambos (and sometimes Kosmas Aitolos and Ignatios of Antioch); this is before we list the lay martyrs.
So there is a degree to which each local Church has leeway to add names to the list, and some of the hierarchs permit some leeway to their clergy as well.