It is surprising* how often the same three Ringwraiths (out of nine, after all) appear in WOTR armies. Each of them has their own GW invented model. While it's fair to say that I am not a fan of any of the invented models over the original look, only the Knight of Umbar (middle) looks other than silly out of the commonplace trio. To my eye, Khamul (left) and The Betrayer (right) do not look much like Wraiths at all.
Anyway, most players actually field the much better looking basic Wraith model, or just choose their favourites out of the invented ones to represent whoever they are taking of the 'Big Three'.
The reason these get taken all the time is because of their bonus special rules. I've already noted that Ringwraiths are a very good deal without them, so it is not strange that Ringwraiths are so popular in Evil armies. Aside, of course, from the undeniable fact that Ringwraiths are cool, you also tend to get say 'Khamul the Easterling' leading an Easterling army, which makes a lot of sense. I take the Witchking of Angmar to lead my Angmar army afterall. Also, it is kind of hard to get around the fact that Khamul is one of only two Wraiths actually named by Tolkien, which matters a great deal to those of us who like the setting. So the answer has to be something other than 'ban Khamul'. Personally, I would like to be able to take Khamul, and have him actually be fair.
However, some of the special rules are really, really screwy. They would be silly on some of the 300 pt heroes out there, let alone the 125pt Ringwraith. 'The Big Three' have the most egregious of these rules.
Without altering points, I have some suggestions. Firstly, I tend to think all 125pt RingWraiths should be mastery 2, this way they are less magically gifted than the Witch-King and the Istari. Also, I have some modifications to the 'Big Three'.
Khamul (on the left) converts 1/3 of all hits his formation takes to automatic kills on another nearby formation. So, 1/3 of all damage is negated and then Khamul's player can pick another formation to take those casualties instead. When Khamul is far away from his enemy, those kills come off something else in his army.... but when he is in melee they inevitably come off whichever nearby unit is in combat with him and would otherwise be the hardest to harm. Picture a unit of goblins being charged by Knights of Dol Amroth. Normally those goblins are screwed, but with Khamul those Knights are at serious risk of being wiped out entirely by the bounce back from their own attacks. The upshot of this is that fights that should go one way, swing about dramatically and the side fighting Khamul tends to lose instead, along with losing whichever were his best units nearby. He is certainly beatable, just a bit too good for his points and way out of line with other characters. On one memorable occasion, I took out Khamul's unit last in the battle. He was pinned to the front by Faramir's brave Ithilien lancers, to the rear by a doughty unit of veteran warriors, then smashed in the flank by Elessar, King of the Reunited Kingdom, and another formation of Knights. The result? Well all of Easterlings died, and Khamul with them. However, his player got to pick where the bounce back damage went. It wiped out the formation of Knights, taking King Elessar with them (hopefully he was merely unconscious at the bottom of a ravine, being given 'the kiss of life' by his horse), which was more of a Pyrrhic victory than I really like to suffer and my opponent enjoyed the bonus VP as well.
Suggested Rules for Khamul: The only other character with a save like Khamul's is the 75pt Elf, Cridan. his is a 6+ and does not inflict kills on other units. Also, Cirdan is a pretty weak caster, one mastery level, one might point and one spell list. I would suggest that Khamul would be more balanced if his own save worked on a 6+ and any bounce back damage occured on a further 4+, effectively a 6/4+ chance of doing the kills, as opposed to the current (4x as bad) 5+. Also, though this might slow things, I would suggest that it would be best that hits that are bounced back on enemies can only go onto the formation that inflicted them, taking away the current freedom enjoyed by Khamul to really punish one unit.
The Betrayer: The betrayer is more straightforward. His rule allows rerolls to all melee and shooting rolls for his formation. Obviously this is a huge lift in their overall lethality. The problem as I see it is twofold: 1. Only a couple of characters get abilities that lift the damage of entire formations. this is a good thing, since it is obviously easily abuseable. Of the characters that do, they tend to just provide a fight bonus to the entire formation, rather than just their company as normal. This is usually +1 or +2 dice per company vs most opponents, so nothing too major. Most of the Damage boosters: Anduril/Narsil, Aiglos, Epic Rage, Epic Rampage, and so on, just apply to a single company. The key exception I can think of is Dalamyr, however his costs might and he only gets two might points. This is not to mention that they come on more expensive characters/or cost might, whereas Ringwraith abilities are free. 2. The Betrayer's ability affects both shooting and melee. A lot of the shooting in this game is already very good and the Betrayer gets to massively improve some, while suffering no risk at all of being dueled.
Suggested Rules for The Betrayer: 1. The rule should just apply to his company. Simple. 2. It should apply to melee only, just like almost all the other damage boosters' abilities.
The Knight of Umbar: The Knight of Umbar takes either (his choice) of a nearby opponent's fight or strength value and applies it to his entire formation. Sometimes this is meaningless, but most of the time there will be at least a high fight hero nearby and quite likely a high strength unit. Imagine a monster's strength being given to an entire formation of infantry. It starts off as powerful as Theoden et al's fight boosting abilities and then goes on up into the mad heights from there. Essentially, it is the fact that applies to an entire formation that provides the ability for it to get out of bounds. Other formation wide abilities (well, except for the Betrayer) are intrinsically restricted to a certain ceiling.
Suggested Rules for The Knight of Umbar: As with the Betrayer, this would be much more reasonable if it applied to only his own company. That way there is far less scope for it to get out of bounds.
At mastery 2, and with these suggested abilities, 'The Big Three' would still be a very good deal. However, they would be less drastic in game and, possibly better yet, we might see more variety in the Ringwratihs that appear.