The followings views are based entirely on my own personal experience and your view might be different based on different heroes combination, and equipment used.
1. Cao Cao (Sabre)
You will use him a lot, probably through the entire Loop 1 of the game. He is not a bad figther, and his skills are useful especially when you do not have the top of the line equipment yet, and your heroes’ levels are still low. I hardly use him in endless mode and not at all in Loop 2, as his skills are not as useful in higher difficulty settings.
2. Li Dian (Sabre)
The most powerful sabre unit in the entire game. His speed is fast, range is wide, and attack power is only second to Xiahou Dun. The only downside is you have to pay to get him (VIP 1). I use him a lot in the early and middle part of Loop 2 as a “cleaner” behind Xiahou Dun or as “softener” before the enemies run into Xiahou Dun. Not as useful in endless mode as well as towards the end of Loop 2.
3. Cao Pi (Sabre)
Statistic wise, he is not impressive at all, with stats almost similar to Cao Cao but lower attack power. Cao Pi shines in endless mode and towards the end of the Loop 2 because of his passive skill — 15% chance of boosting the speed of friendly units nearby. Cao Cao can do it too but it consumes energy, and needs manual interference. It is quite a sight if you have many other units around Cao Pi, and see how fast the enemies perish. His other skill of repairing the tower health is also invaluable in Loop 2, and in endless mode, it will mean lasting another couple more waves.
4. Dian Wei (Sabre)
Seriously speaking, I wonder how many players out there actually put him in the permanent roster?
5. Xu Shu (Sabre)
You get Xu Shu only after completing Loop 2 of the game (any difficulty), which is very very late. By then, you are probably exhausted from the game, and not wanting to play the game any more. His passive skill is very interesting though, with chances to slow the mobility of enemy units, which is really helpful in higher difficulty settings.
6. Xiahou Dun (Spear)
The most powerful hero you can get in terms of raw attack power. Armed him with the Blue Dragon Sabre & Imperial Seal, and he is unstoppable. Xiahou Dun is my main damage dealer for the entire Loop 1 & 2.
7. Zhang Liao (Spear)
It is a tough choice between Zhang Liao and Xiahou Dun. Zhang Liao has slightly lower attack power, but higher speed, and better range. Personally, I think it all boils down to who else are in your permanent roster, and how are you going to equip each of them. In my game, the range difference is negligible, as Xiahou Dun’s range already overshot the path most of the time, what is the use of added range?
8. Yue Jin (Spear)
As with Li Dian, you gotta pay in order to get him. Yue Jin is very useful in early games as his starting stats is the highest among the spear wielding heroes. His skills are interesting as well, but personally, Yue Jin appears in my games only as a secondary spear man, mainly because of his low range, as I have used all the best range boosting gears on my mage.
9. Xu Huang (Hammer)
I couldn’t comment much as I never use him.
10. Xu Chu (Hammer)
I prefer Xu Chu over Xu Huang because of his passive skill- a chance to stun enemy units. All in all, as much as I would love to put Xu Chu in my permanent roster, unfortunately there is no more room in my party, and would use him only in maps where there are no flying enemies.
11. Jia Xu (Magic)
Jia Xu has the highest attack and speed among the 3 mages in the game, but his range is pathetic. He might look weak, but in actual fact, you will probably grow to like him in Loop 2 where resources is scarce (if you do not use money bags), and he is the only mage that can take on the first couple of waves by himself before you have enough money to plant your main damage dealer.
12. Guo Jia (Magic)
Guo Jia is my favorite mage. In my own opinion, his skills are better than Sima Yi, and he is available much earlier and cost much less. The ability to slow and halt all enemy units’ movement is priceless, and save my life in many endless and story mode games, furthermore, his passive skill of absorbing more souls is very helpful as well.
13. Sima Yi (Magic)
Sima Yi has almost identical stats as Guo Jia, but a wider range. As with the choice between Xiahou Dun & Zhang Liao, it is a tough choice between Sima Yi & Guo Jia. I would think it again boils down to personal style of play.
14. Xiahou Yuan (Bow)
At a price of 450 gold, Xiahou Yuan is a steal. Xiaohou Yuan has the highest attack power but lowest range among the 3 archer heroes, but his usefulness does not lie in his stats. It is his skill of 10% chance of instantly kill a flying unit, rallying all archer units, boosting their stats in the game that makes him a permanent fixture in all games except maps with only land units. Flying units in FOW are the most irritating units in the whole game, especially in Loop 2 where they come in hordes, and Xiahou Yuan will make your life a lot easier.
15. Zhang He (Bow)
He is useful before you recruit Zhen Ji, and is completely overshadowed by her later. Though his attack power is higher than Zhen Ji, but his skills are really not helpful at all. Still he is useful as a 3rd archer for maps where there are a lot of flying units.
16. Zhen Ji (Bow)
At a price of 9999 gold, she’d better be good, and she is. Comparing to Zhang He stats wise, she has the same range, slightly higher speed, but slightly lower attack power. Skills wise, she is a goddess, all her skills are very useful in endless and story mode whether slowing enemies, or boosting friendly units stats, or stunning enemies, or instantly reduce all enemy units life force by 90%, man, I wonder is there any players out there could pass through some maps in Loop 2 at highest difficulty level without her in the party? Having her in a party is like cheating.