I've only ever played the Western version so my only experience is with Hard Mode, though I'd assume similar arguments apply in Maniac from what I've seen of lets plays of that difficulty.
For a tier list like this I personally just ask myself the question 'if I had to run an iron man and got to use only one of the two units I am comparing, which one would I use and why?'. With that in mind, my personal tier list would be :
1. Oscar. Yep, he's better than Titania in the harder versions of the game, and not by just a little either. Titania is noticeably more frail due to lacking his broken earth-earth support with Ike, and has worse defenses than Oscar as well. She will hit slightly harder through much of the game, but in the harder versions of the game it's more important to be able to tank through a ton of damage while you grind away at the hordes of enemies facing you than it is to one-round enemy generals. Losing Oscar in a hard mode / maniac mode run not only eliminates your best mid-game front-line unit, but also makes the rest of your front line decidedly more prone to dying (Ike and Kieren badly need his support to survive). He falls off a bit in late game but not nearly as hard as the other paladins do.
2. Titania. I actually am not a huge fan of Titania (sacrilege, I know), in the sense that she is best at a point in time where I don't feel I need her. The early chapters aren't too difficult on hard mode, but the reason she still sits at number 2 on this list is that chapter 7 and non-cheese chapter 8 are incredibly difficult without her. Prior to getting access to the base she's just so far and away your strongest unit that losing her would hurt worse than, i.e. losing Ike in the final few chapters - though only by a little, I think. She becomes nearly useless once you hit like chapter 23 or so, as she stops OHKOing enemies and becomes too frail to be on the front line even at max level. The later chapters are longer and more difficult, so personally I consider her usefulness as being invaluable for 1/3 of the game, incredibly useful in the middle third, and then completely replaceable / borderline useless in the last third.
3. Ike. This is where I'm probably going to diverge from most of you, as this forum seems to have some fetish for claiming Ike isn't a tremendous unit. I get that everyone is trying to be hip and edgy but the reality is that while Ike noticeably lags behind in chapters ~12-17 or so, he is your best front-line option outside of Oscar or Titania in the early levels, and is absolutely indispensable in the later levels. Additionally, even when he is at his weakest, he can provide invaluable dodge supports to the rest of your frontline. If you take Ike out of your team as a combat unit then you will struggle in every phase of the game outside of a few chapters in the mid-game where paladins can run rampant, and the late game chapters will become impossibly difficult in the harder modes.
4. Marcia. Flying utility is amazing, but in the hardest modes of the game her combat starts to be a little suspect. She can't OHKO many enemy units, and if she isn't using the full guard then she will get one-hit by a variant of enemies (and if she is using the full guard then she can't be using something like the fighter band to buff her strength growth). That being said, there are tons of strategies that are impossible without a buff Marcia or Jill, and there's a reason that she's generally first pick in LTC or speedrun drafts. For this tier list I'm primarily considering iron man runs on hard / maniac modes, however, and I just don't think that Marcia really shines in safer playthroughs on harder difficulties. Also she has worse availability than any of the above units, as she can't help at all in the early game and will take a chapter or two of the early-mid game to get going.
5. Jill. Basically the same as Marcia except she's harder to get going and get doubled by a lot of enemies early on, which makes her difficult to front-line with. She has worse availability than Marcia by a few crucial chapters as well.
6. Reyson. Going through hard (or maniac) mode without Reyson sounds really awful. I've never tried it, admittedly, but he's such a great insurance policy that trying to run an iron man without him sounds incredibly taxing. Also joins roughly halfway through the game.
7. Kieren. I know a lot of people love this guy (and I do too, honestly - he's so much fun to use!) his hit rates are pretty bad on harder difficulties and he doesn't have flying utility to make up for joining later than the top-tier picks in slots 1-3.
8. Soren. His offensive prowess, tremendous growths, and long-range tome and staff utility make him useful throughout the game, but man does he suck in the early game - he is actively a liability if you're trying to train him for a few chapters, and takes way longer to get going in the harder difficulties. Additionally, he can never front line at all and has relatively poor movement, meaning he gets left behind on a lot of mid- and late-game chapters. A lot of the early chapters are set up for him to sit behind a front line and take kills, though, so in some sense the game pushes you to use him and he is easy enough to use even with mediocre growths. I will still use him in every single playthrough since he's my favorite character, but I'm not going to pretend that if it were an iron man I'd want him on my team over any of the A tier units.
9. Lethe. She's really useful in tougher difficulties as her gauge is tremendous, she has great movement that can keep up with your paladin front-liners, and she can tank a lot while transformed. Laguz in general are pretty bad in the longer fights that hard / maniac mode cause, however, and so in that sense she needs other front-liners who are strong to be able to hide behind while she is untransformed.
10. Boyd. He suffers a bit in hard mode as compared to normal, but he has a lot of potential as a secondary front-liner who can tank a hit or two but not five or six. He is with you the entire game, uses the best weapon type in the game, has good supports, and lots of offensive potential to help supplement the tankier, lower damage options you get like Oscar or Ike (pre-Ragnell). This pick vs. Astrid, Gatrie, Tanith, or Makalov was pretty tough and I'm open to changing this since I think there is a decent drop-off after Lethe.