CT endings list (no ending-spoilers)

#3. TDP - defeat Ocean Palace Lavos -OR- use the NG+ Telepod before first-ever visit to 600AD
#2. RE - anytime after Black Bird, without Crono revived. (Variations: Epoch crashed, or use EoT/Telepod)
#1. BT - anytime after Black Bird, WITH Crono revived. (Variations: Epoch crashed, or use EoT/Telepod)

#4. TSoG - after Yakra [use NG+ Telepod], before Trial (which bars you from reaching Telepod)
#5. GN - after RSeries [use EoT], before visiting 600AD
#6. TLH - visit 600AD after Heckran [just in-and-out, use EoT], before Masa&Mune boss..
#7. TUP - get Hero Medal after Masa&Mune [use EoT], before partying with Ayla (which takes away Gate Key)
#8. PotT - after Nizbel [use EoT], before Frog joins again
#9. TO - after Frog joins [go up to Truce gate, use EoT], before Magus
#10. DA - after Magus [go up to Mountain gate], before Black Tyrano
#11. WTPS - after Black Tyrano [return through 12000BC gate], before seeing Schala open the Sealed Door
#12. TSS - see Schala open the Sealed Door (center hall of Zeal Palace) but DO NOT charge pendant: leave Zeal and climb back down to 12000BC gate [use EoT]. This ending is the easiest to miss.
#11. WTPS - after Schala seals 12000BC gate [use EoT], before Ocean Palace. (This is the 2nd window to access Ending 11.)

Note: Once you have access to End of Time (by Ending 5), you can defeat the Lavos Shell, return through the gate, and then continue your endings-collection quest with the Shell permanently defeated.