Um...honestly, if you've tried multiple times and haven't been able to get into the games, then I suggest just giving up on them and moving on. But, for the sake of discussion, I shall attempt to give you some pointers. Er-hm...
VII.
Well, discussion is done. Moving on.
Kidding, of course. Let me see if I can break this down...
I would not suggest starting with V. It is very bare-bones I feel, at least in terms of story. This is back before epic tales were spun by Final Fantasy. I hear VI is the best from a lot of people, but I have never played it, so I can't speak for it.
VII. It is the most popular by far. In fact, it is so popular now that many people will tell you to stay away from it because it's overrated, it's stupid, the characters are annoying, blah blah blah. It is good. The story is long, the combat is solid (assuming you like turn based), the characters are actually very well written with more than just a shallow personality cliche, and there is so much more to do in the game than just the main quest. This game is massive, but in a good way. It's not just filler fetch quests or collecting stuff. Everything really does have a purpose, or is at least mindless fun.
VIII. Personally, my favorite. It was the first one I played, so it has a special place in my heart. It is certainly different from the others in the series though. Much more sci fi than fantasy here, with motorcycles, flying cities, machine guns, and so forth. Also, before X, this is the game that looks the most realistic. As in, the characters are super deformed or anything. A lot of people think this game is the black sheep of the series, and that the Junction system was crap, but I think they're wrong. If Sci Fi is more in line with your tastes, I'd say give this one a try.
IX. If you want to see what Final Fantasy was before VII, give this one a shot. When it came out, it was hailed as a return to form for the series. Cute character designs, a heavy fantasy setting, with knights and castles and an evil queen, and sky pirates and all that stuff. If classic Japanese style RPGs are to your liking, this is probably the best one to start with.
X. Honestly, find out if you enjoy another one of the games first. X is Final Fantasy through and through. It has VIII's character design (people look like people) but it has the fantasy elements in there too, such as riding giant birds and people relying on magic instead of tech because "tech is bad!". If you liked any of the previous ones, then you'll enjoy this one. Or, you can just jump right into this one. It is probably the best mix of all the previous games and strikes the right balance. If you don't like this game (and no, not liking it because the voice acting isn't what I'm talking about), then Final Fantasy as a series probably isn't going to be for you.
XI. Never played. MMO therefore not my thing.
XII. Ugh. Look, people say XIII was the low point of the series. I think XII was, but I tend to be in the minority opinion. Everything that makes a Final Fantasy game a Final Fantasy game--great characters, epic story, expansive world, fun side quests--was just missing from this game. The plot was...I don't even remember what the plot was. All I know is that it relied heavily on politics, which is not what I want from a FF game. The fact that every character could use every weapon made them all bleed together in my head and I still have trouble remembering them. No one had any great revealing side quests or backstory. The only real side quest in the whole game is hunting down beasts, which gets real boring, and to top it all off, the game can literally play itself. If you set up the right commands, you can set the controller on the floor and just watch as your characters do the work. That's just...ugh.
XIII. Okay, this one gets a lot of crap. And rightly so. Yes, for a good chunk of the game, you are going to be running down a straight path. Yes, the characters aren't as great--and Hope is downright annoying--as they should be. Yes, the plot is kind of dumb, even for a Final Fantasy game. But with all that said, I STILL think this game is better than XII. At least I can remember who these characters are, what their motivations are for doing things, and what the plot of the game is about. To me, this was a step back in the right direction for Final Fantasy after XII completely derailed it. Is is perfect? No. Should you start with it? Heck no. But if you have to choose between XII and XIII, I'd vote for XIII.
Here's a bonus one. Tactics. A lot of people love this game because of its in-depth job and battle system. They also say the story is amazing too. While I did enjoy it, I still don't really understand what went down in that game beyond the basic "civil war is going on in the background, but you are trying to stop a much larger threat.". If numbers and strategy are more to your liking, and story isn't really that big of a deal, this one might be worth a try.