Re: Most Useful Units
There are many good units on the game, but I think most useful ones are those you can recruit since the beginning (due to availability, cost and easy retrain), in particular:
Bow Quendi are awesome, ok their recruitment cost is quite high, but they are very good archers (long range, good damage, very good stamina, high morale, good precision, high speed) and they work fine against 90% of regular infantry.
Much similar to light eleven archers, but I found them to be slightly better; plus, in the very beginning of HE campaign if you are on blitz you'll probably get a lot by "send reinforcement" missions, which will leave you with only the upkeep to pay and a 1/2 stack of Mg42s.
another excellent unit is dwarven warriors (coupled up with axemen asap), they are cheap compared to other units with similar stats, easy to recruit and available in large number; they can't of course, win battles by themselves, but you can trust them to always hold the line and they become super when holding narrow places.
Axe throwers, if used to soften and flank enemy formations are very useful too.
More in general, any mid tier unit which is recruitable since the beginning (usually units specific of a region, but not always) like Helmlingas, Battle Ready Dunedain, Guards of The Orthanc, Morannon guards and so on often proves to be very good and useful units
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