1) Hyperion, Araji, Eden (a.k.a. Super Titans) – if you have these rare titans they should be leveled first. Best titans for guild wars AND the dungeon. Hyperion is able to heal (the only titan that can), and Araji generally has the best attack.
2) Angus, Nova – these are the best two titans aside from the rares above, and necessary for guild wars as they round out your “standard team” of 5. Nova is more critical for guild wars and dungeon, although Angus is also important as the Earth team tank.
3) Ignis – he is the best titan to substitute above for guild wars and dungeon if you are missing one of the 3 super titans. Some people even use a team of 3 supers, Ignis, and Nova when defending. If Ignis fires his ultimate at the right time, Eden can do over 3 million damage at once, so be careful when you see someone defending with Ignis. On average, a team of Nova, Ignis, and the 3 supers will defend successfully anywhere from 10-25% of the time vs the “standard team” of Angus, Nova, and 3 supers.
4) Moloch, Sigurd – these are the two remaining tank titans and will be crucial in the dungeon. In addition, you can occasionally use one of them in guild wars if you are missing Angus.
5) Mairi, Avalon – somewhat useful titans in the dungeons only as tanks for your mixed team. Usable in guild wars along with Nova, Angus, and Ignis when you have zero super titans.
6) Vulcan, Sylva – the two worst titans in the game. These should only be leveled as last priority.