Ike: Haar is always going to be at least five levels higher than him as he basically has 2-P all to himself and 2-F is filled with exp, while 3-P and 3-1 have less enemies and more viable units to split the exp between. Once Ike hits level 20, that level difference will only get higher. When Ike gets stuck on Level 20, Haar (--/20/5) will have +4 HP, +5 Str, = Spd, +2 Lck, +6 Def, and +6 Res, not to mention +2 Move and flight, as well as a 3rd tier skill that has about a 30% chance of activating. Haar's flight is useful in 3-2, 3-3, 3-4, 3-7, and especially 3-11, and his higher move and Canto is useful during 3-8, as it helps him better avoid lava tiles.
Ike gets Ragnell near the end of Part 3, and even then, Haar will be doing more damage with a Steel Poleax (which, because of his high skill, he can use without fear of missing), and Haar only widens the statistical advantage he has over Ike with every level-up he gets, until Part 4 rolls around and Ike finally promotes. Haar is of course going Micaiah's route for the desert chapter, in which he is the single most useful unit you could possibly bring into that chapter (Naesala is a close runner-up but has to run away from the several Crossbow-weilding Snipers on that map). Ike, while statistically strong, is going to be underleveled and thus not too much better than most other units you've managed to train to that point. He's definitely not the single most useful unit you could bring in to either of his Part 4 chapters.
When the Final Chapter rolls around, Ike will likely have a speed advantage over Haar, although it won't be too great because Ike still isn't --/20/20 while Haar is. Haar will still be able to double all the generals in 4-F-1 and his flight allows him to easily jump back and forth from the two wings to the left and the right of the central path, as well as get around the enemy units who like to clog up those pathways and get to Lekain/Hetzel quicker so they don't pull out their obnoxious Silencing effects. In 4-F-2 Ike is trapped in the Black Knight's room, which basically makes him useless for the actual combat portion of the chapter. In 4-F-3 Haar is also doing more damage than Ike due to Urvan.
I fail to see one part of this game where Ike is superior to Haar.
Reyson: Reyson, untransformed, at base, has 11 speed, 6 defense, and 40 HP. So, perhaps he isn't getting OHKO'd, but he's definitely getting one-rounded. Until he transforms he only gives one unit and extra turn. He's only in six chapters before the final, and in his Part 4 chapters, he will have a difficult time catching up with Tibarn and Elincia. Why, then, is he so damn high? He's helpful. He can, in the right situations, provide an irreplaceable service that greatly benefits your team. However, for most of Part 3 Haar also provides an irreplaceable service that greatly benefits your team, only Haar isn't a liability and is also around about twice as long as Reyson. Having Reyson as the second-best unit in the game implies that using him instantly makes the game easier. I don't see how a unit who constantly needs to be covered and hidden from any possible enemy makes the game that much easier. As for Laguz Stones, you get a mere three in the game, if Serenes' information is correct. I fail to see how making it so Reyson isn't doubled but still 2HKO'd gives him an exclusive right to those, when they could be given to, say, Ranulf so he isn't a useless boob in all the Part 3 chapters you're forced to bring him in on.
And yes, I meant that Gatrie can take much more damage before he dies. >.>
Volug: In Part 1 at base, Volug has 25 or 26 MT (not sure if the halved transformation bonuses of his skill round up or down). Base Sothe with an Iron Dagger has 23 MT, +2 MT for his auto-support with Micaiah, giving him 25 MT. Volug has equal speed and defense as Sothe, too. All Volug really has over Sothe his high HP and movement. However, Sothe is around in 1-2, 1-3, and 1-4, when Volug is not, and in 1-6, you get Zihark, who is faster and stronger than Volug, and Tauroneo, who is much stronger and much more defended than Volug. Volug's only real use in 1-6-2 is charging to the right side of the map to perhaps save a couple more of the NPCs. Volug is only really the best Part 1 Jeigan in 1-5.
In Part 3, unless you're waiting for Volug to transform on his own without Formshift, his attack is not 36, even with an S-rank strike. His attack speed is only 17 or 18 with Formshift, and his defense only 14. Of course, if you know of some way that Volug can easily transform without Formshift, please let me know, as it would void this whole argument.
I'll concede Soren and Ilyana, you guys have me beat there, and Pelleas/Oliver are too close for me to really waste my time debating over, so I'll concede that one as well. But the other three: Ike, Reyson, and Volug, I'm going to continue debating.