Final Fantasy XII

Belias, the Gigas
The first Esper that you encounter in the game – you must battle and defeat Belias in order to obtain the Dawn Shard.

Mateus, the Corrupt
The second Esper that you automatically encounter while completing the story – you must defeat Mateus at the end of the Stilshrine of Miriam.

Adrammelech, the Wroth
The first optional Esper which can be fought in the Zertinan Caverns which can be accessed via the Ozmone Plain.

Cúchulainn, the Impure
Another optional Esper that can be defeated after you have reached the Ozmone Plain for the first time.

Zalera, the Death Seraph
Returning to the Barheim Passage after you have reached the Ozmone Plain using the Barheim Key and defeating the third optional Esper.

Exodus, the Judge-Sal
An optional Esper that can be found at the top of the Mosphoran Highwaste which can be challenged after you have reached the Phon Coast.

Zeromus, the Condemner
An optional Esper that can be fought after you have defeated Judge Bergan at the top of Mt Bur-Omisace (but which is an extremely difficult fight that should be completed later on in the game).

Shemhazai, the Whisperer
A non-optional Esper that is fought in the Ancient City of Giruvegan as you leave the Great Crystal.

Chaos, Walker of the Wheel
Completing the Three Medallions side quest to gain access to the final chamber of the Necrohol of Nabudis to fight Chaos, an optional Esper.

Hashmal, Bringer of Order
Another non-optional Esper that is fought at the top of the Pharos of Ridorana.

Famfrit, the Darkening Cloud
Famfrit is fought during the regular story when you encounter Doctor Cid in the Pharos of Ridorana.

Ultima, the High Seraph
An optional Esper that can be fought after completing the events at the Ancient City of Giruvegan by returning to the Great Crystal, although it should be saved until later as it is a very difficult fight.

Zodiark, Keeper of Precepts
The last optional Esper and the most powerful one. The battle to obtain Zodiark in the Henne Mines is extremely difficult.