I voted for the poor under-appreciated XENOSAGA,
I'm not sure how I can even compare Xenoblade battle system, it's not turn based, I will never play that game probably also thanks to this fact. (can't compare also because haven't played it XD) (also anyone noticed that Xenosaga 3 graphics on PS2 is far better than Xenoblade in later gen console?)
(please understand I'm not 100% sure about the things below, some time has passed since I played those 4 games)
Xenosaga titles and Xenogears comparison:
Battle:
Xenogears: BAD battle system, terrible combo system that you need to learn buttons to press in a guide, everytime can't see the difference between the different attacks, no turn flow display, NO BOOST to change turn flow, ether not very useful, robot battles with max to 3 robot that is good, robot battles are better, the game is so easy that understanding nothing about turn flows and attacks (tha game want you to know nothing because little information is on screen) in general is not a problem to finish the game, that's a little pathetic (I had problems only when battling Deus because in PS2/PS3/PSP shows a game braking bug that doesn't show in the original PS1 hardware.
Xenosaga 1:very good customization (equip and skill) also for AGWS and I like you can have three of them together battling (no ether drive with all of them), different CHARGEABLE BOOST bar for EVERY character.
No Break bar for enemies, AP for characters to charge strong attacks.
Xenosaga 2: best battle system, very challenging, difficult to master, don't like very much character style changed for more western type appeal, customization good only because can change copilot in E.S. (big AGWS with 2 people inside) to change awaken of anima special attack, can have 3 E.S. but only two in battle at the same time, only one can use ether, Zebulun. COMMON BOOST bar for characters. (not E.S)
Combo to get enemies to break zone. Only game where you can move character behind enemy during battle. Ap bar present.
Xenosaga 3:simplified battle system with no weak point combo like Xeno 2, break bar present, different attacks to target HP or break bar, in this game it is very easy to correctly use all the different elemental attacks (very important in boss battles). COMMON BOOST bar for characters, No AP at all for characters. Can use 3 E.S. together! No turn events that cycle things as: +exp, + critical, + money, etc-, good character/E.S. customization.
Winner: Xenosaga 2 (characters battle) (Xenosaga 3 is better for robot battles, Xenosaga 1 is better for character customization -for battle purposes) (Xenogears is simply bad at this.)
OST:
(Xenosaga 3 has some traks that are remix of Xenosaga 2 traks, cool to have different versions!)
Sorry not sure about this one, probably Xeno 3 and Xenogears take the best spot together.
Characters:
Probably Xenosaga over Xenogears but by a little.
Best character:
can't decide between Elhaym or Kos-Mos (aesthetic)
about personality can't decide between Fei and Shion
I like more xenosaga because there is space travel and you see a lot of places freely (if you want to backtrack to get some things) the disc 2 of Xenogears is a JOKE about this 
(thanks U.M.N.!)
Bad exploration in Xenogears for me.
About the story there are moments where all Xenosaga games are silly, but also present in Xenogears, they have some things in common, too subjective to give a better description (battle description is very easy for me), I will say Xenosaga as a whole has better story (but it is unfair, three games against one..)
Bottom line: Play Xenosaga, a lot of people didn't play that, it needs a try! (boost to change order in a turn based combat game is awesome!)
Xenogears to me is an unfinished game, disk 2 is...bad storywise, they ended the money.
Persona 5 on PS3, I won't need next gen!
