Subjective but in my opinion, FF12 summon designs > FF13 summon designs

http://images2.wikia.nocookie.net/__...lias-FFXII.png
http://images3.wikia.nocookie.net/__...3/3e/Chaos.jpg
http://images1.wikia.nocookie.net/__.../cc/Zalera.jpg
http://images4.wikia.nocookie.net/__...cb/Zeromus.jpg
http://images4.wikia.nocookie.net/__...1f/Hashmal.jpg
http://images4.wikia.nocookie.net/__...ers_ultima.jpg
http://images4.wikia.nocookie.net/__.../3a/Exodus.jpg
http://images1.wikia.nocookie.net/__.../f/f2/Cuch.jpg
http://images2.wikia.nocookie.net/__.../Shemhazai.jpg
http://images2.wikia.nocookie.net/__...drammelech.jpg
http://images2.wikia.nocookie.net/__...rs_famfrit.jpg
http://images1.wikia.nocookie.net/__.../18/Mateus.jpg
http://images1.wikia.nocookie.net/__...d7/Zodiark.jpg

If only FF14 Zodiark was a esper and not a boring crystal...