Here's my list of the most valuable and east to get L in every books imo (not only for war, it's a general review) :
Dragon : G. Darmith -> General trait, easy to get, one of the strongest dragon and fire element in the same time, definitly worth to spend a few war coins
Spirit : I would probably say Singularis as a magic L, for the good speed and the paradox skill or Glitch for the possession immune trait and good possession skill. I'm personnally not a big fan of G. Uria, but he's cheap and east to get, if you really need someone asap.
Winged : Lot of choice here, G. Ingvar is the safest pick, reliable light general with good defense. Caillech is op as a speedster and Lilysha is a good choice too since she's got 2 possession skill, one stun, decent dammage and is also in the female chapter !
Sea : Tons of choice : LotA as the easyest L to get, since he's breedable. Varuna as a polyvalent speedster (but got outspeed by a lot of other denyer now)... If you only need one, obviously go for G. Thetys (100% hit aoe stamina leak for the win). Also note that Volray is really good as an Epic for team wars. This guy's little voltaik with a 75% stamina drain skill.
Underworld : Vadamagma lvl 130 really does the job, and is breedable so no excuse ! Lot of op mob to get here : Metalhead, Ouros, Lilysha, but I personnally run with my favorite of all time : Barbatos. Please make me the favor to get this guy as soon as you can !
Mecha : Again lot of choice, but the best one's definitly Timerion. Mechamancer's good too with 2 possession skills. I also like Exo skill with the 50 dmg aoe skill, single stun and easy to get since you can have it in adventure map. If you don't have any other thunder L, he does the job for a while !
Female : It really depends of what elements you need, since the goal is to have at least 2 max lvl L of each element / chapter now. If I just look at that chapter, not at element I would say the female worth to get are G. Thethys, Caillech, Sarah, Lilysha, and Shalinar (which is to me the best fire L right now).
Undead : This one's hard, I would say Darkzgul if you really don't have anyone else (since he's breedable and got up a bit recently). I personnaly run with Count vlad, polyvalent dude with decent stat and possession skill.
GLegions : Good luck with this one, it's still a mystery to me... I like Thorder lvl 1xx with 3 speed runes since you can outspeed a water denyer and still one shot him/her before he/she freezes your team forever. Btw this guys is breedable, ez pez. Needless to say I don't really like General pets but if you need to chose one, maybe Uria's pet, with the 85 single dmg and decent speed.
Evil Legions : Nebotus if have no L yet since he's breedable. I personnaly run with Draza, who is extremely violent with some strenghs rune and got nice skills, including 2 stuns.
That's all for me. Please note that this list is non-exhaustiv, all of the L have some strenghs and weaknesses, my goal was only to point out the best L of each chapter, what ever your level in game is