It also depends on your race,and where you are in the tech tree.
Untill the end of chapter 10 Barracks + Needles
Chapter 10 Barracks + Needles and Mercenary Camp + flying academie, upgrade you flying academy untill it has the same level as your needles:
Interesting units: for human; Hunter, Frog (once the final powers are unlocked at the elvenar chapter), Elves: Flower Girl and Watcher. both as soon as 2* versions are unlocked. must haves.
Chapter 13: Training Grounds + victory springs, upgrade victory springs untill its level is the same as needles and flying academy
Interesting units: Both for elves and humans the dogs
Unfortunatly the only usuable unit for both races in the training grounds is the dog, it's an awesome unit. Orc Strategist is also an awesome units, unfortunatly it requires orcs to produce and that is a problem. its better to make them using the terrein of the orc strategist from the magic academy crafting recipe pool. you can craft these multiple times.
The Mercenary Camp is awesome, for elves very early in game and for humans very late in game.
Hunter: this unit at least matches the elves bow unit, and can be considered surpassing it once it reaches 3*. 4 moves works excellent in it's favour to always hit first. it massively better than the 3* crossbowman from the human arsenal.
Flower girl: The elves magic unit just sucks, at 1* this young lady is very flimsy, as a 2* unit it easily surpasses the elves unit, at 3* it's awesome, for humand this unit is pointless as it's priest is in most cases just better then this young lady.
Watcher: excellent unit expecially for elves, the no range from the treants are often a problem and lead to random losses if the terrein doesnt suit it's needs.
it's 1 range makes the outcome more predictable, the 3* version of this unit is on par with the human paladin making it a choice depending on the enemy, somtimes the paladin is better other times the watcher.
Frog: Nice niche unit for elves, there golems are already strong and powerfull, but there are matchups where this unit works wonderfull.
On the other hand this unit is a must have for humans, the 3* version of this unit (2* sucks) is exactly what humans need in there arsenal, there a lot easier to produce than the strategist, while packing a powerfull puch due there special power, its human counterpart totally sucks and is a horrible unit because it has the wrong 3* power, this unit fixes that problem. it's esentially a mortar but then with the correct power.