To be honest with you, my favorite part of Final Fantasy XII was obtaining Espers. I loved it. I loved how each had it's own conditions and hunting it and finding it was so unique. I have to admit that once I got them I did not use the earlier ones in battle as much, but the adventure to capture them all and get them to side with my party was pretty thrilling.
Attempting to get Zodiark in that final dungeon was a pain in the ass. I spent countless hours leveling up and whittling my way back there only to find no save point and die deep into the Esper battle. I did obtain it my 2nd attempt, but I broke my DualShock 2 controller in the process after my 1st defeat.
I agree, getting all the Espers was one the things I liked the most about FFXII. Ultima and Zodiark were both a pain to get, and yet the feeling of satisfaction is amazing after you collected them all.
FF games played so far: FFIV; FFVI; FFVII; FFX; FFX-2; FFXII; FFTA; FFXIII Trilogy
Currently playing: Kingdom Hearts Re: Chain of Memories (PS3)