Theodred is similar to Gothmog.
He is not as resiliant and does not have as much might.
This is offset by his Hero of Legend special rule that gives him a chance of regaining lost might.
In all he is very good hero.
Theoden is also a very good character.
He is a leader of troops rather than a character killer, so you still need to be careful with him.
His Heroic Exmaple ability is particularly useful when defeated in combat to taking a terror test.
His Touched by Destiny ability ensures an ample supply of might.
Deorwine's In Defence of the King special rule makes him invaluable if you are playing Theoden, Theodred or Eomer.
Like Deorwine, Meriadoc is a supporting character.
He helps ensure that the formation is steadfast.
He is particularly effective when he is near Theorden.
Eomer is Rohan's most effective character killer.
He has both Epic Challenge and Epic Strike.
Eowyn is very useful at killing minor characters.
Her Disguise ability allows her to get close to a mage undetected.
Erkenbrand is rather inexpensive and has plenty of might.
Eorl the Young is an all around good character.
He can kill enemy heroes in a duel thanks to his epic challenge and strike special rules.
He is also an Inspiring Leader and can call the ever important Epic Charge.
His hero of Legend special rule ensures that he has plenty of might.
At first glance Elfhelm's Riders appear to be expensive versions of Riders of Rohan.
I assure you that they are worth every penny.
First, they are faster than regular cavalry.
More importantly they have the Pathfinders ability which allows them to move through terrain unhindered.
Grimbold's Helmingas are more expensive than regular infantry.
For this they gain some strength.
However, their low defence makes them difficult to use.
Erkenbrand's Riders are good, but I prefer taking Royal Knights or the Sonds of Eorl.
The main reason to take The King's Guard is to have Theoden and Eomer benefit from the might saving power of the Royal Standard of Rohan.
Thus it is best used in large games.
I am not a fan of Theodred's Knights, as I prefer taking Theodred as an Epic Hero.
Further, Royal Knights are almost as effective and much less expensive.