Damn, I want to re-start X now.
Out of those choice, I'd say X is my favourite, but overall its X-2. This is because not only did it let your characters all do different things, but because some of the jobs were so entertaining (try a battle with 3 Gamblers.)
My thoughts on 7-13's are below. As a note, I'm a massive grinder in JRPGs (one of the reasonsI like them). I have been known to find a good levelling spot, stick some music/TV show on my PC and spend the afternoon fighting the same 3 battles over and over.
VII - I didn't like the game overall, but I found the Materia system a bit too overly complex and it was annoying having to switch it between various characters. I do like how it levelled up as you used it and if they were to bring something similar back in a future game, I wouldn't disagree.
VIII - I liked the idea of the system (and the ease with which you could switch GFs & Junctions between party members), but it was too easy to abuse. I once spent an hour fighting one battle against 3 enemies at the beginning of disk 2 getting 99 of each magic each enemy had for every character. I could then just plow through the rest of the disk without doing anything. It also discouraged you from actually casting magic which seemed a bit of a waste. The GF's were cool and levelling them up to get extra abilities and was a nice touch that should be repeated.
IX - I liked how everyone had a pre-defined with their equipment improving them and supporting them, and I thought the AP system letting you choose extra buffs was also a good idea. It also encouraged grinding regularly to make sure you learned everything. However, the artificial barrier of having to wait for the next equipment to continue was annoying.
X - Probably the most well balanced system they've ever managed. If you don't grind, you get enough spheres and xp to get each characters final ability before the final boss and while the boss may be tough, its do-able. However, you can grind like a madman and not run out of new things to get. It also encourages specialission at the beginning before then getting you to start to mix and match. However, if you have the time, its open to abuse. My friend had a save game where Yuna could hit an enemy in Omega Dungeon for 24k damage.
X-2 - I love that its the job system with a bit of a twist. The fact each job levels up as you use it encourages specialisation and some of the jobs were great. The bad thing is it didn't really courage you to diversify once you got a routine set and the additional mini-sphere grids were often a bot of a waste as they encouraged you to switch between jobs in battle when its never really needed. Its probably my favourite, but its a pity the system has such a bad name and the game overall is fairly hated (I like it though).
XII - I really hate this system. The first reason is a lack of specialisation. In X, even though each character could learn everything, it would take absolutely ages and even then, they had unique weapons to use. In XII everyone could end up learning to use all weapons and all spells and abilities meaning you just ended up with characters whowere all pretty much the same. The second is if I've spent time grinding XP to use a spell or ability, I shouldn't then have to wait for it to become available and then spend gold to BUY it. The third is that the grind was too easy. There was a system where if you killed the same type of enemy, you would get an LP bonus. About 1/2 way through the game you came to a forest where there were a lot of the same type of enemy and this coupled with the Gambit system (so you don't even have to press any buttons in combat) meant that after a couple of sessions, I had everybody's license board maxed out. I quit soon after.
XIII - I like this because it encourages (ok, forces) specialisation with each character getting different abilities for each job early on (One gets defensive buffs as a synagist early on, where as another gets offensive buffs instead) and by the end everyone who has a role as one of their primaries get access to most abilities. I didn't like it because of the artificial walls to levelling (can't access the top without beating the game? BAD IDEA!!) and the combat system means a lot of abilities don't get used. It was also well balanced if you wern't a power-leveller which was also good.