First generals path guide

The generals system for the campaign mode has been changed in GOG3. You can now only use the generals that historically fought on your side of the war, meaning you can't use allied generals in axis campaigns and vice versa. Thus you can no longer defend the beaches of Dunkirk with Guderian. An exception to this however are de IAP's.
Besides this change, for each side of each theater, there are a select number of generals, (lineup) that have a chance to gain medals when attacking enemy units.

Because of these changes it is now vital to chose two general lineups, one for the axis and one for the allies, while also keeping in mind with which generals you can farm more medals in campaign.

Like previous Easytech games, you get a handful of free generals. Each theater, as far as we know at the moment, gives 2 to 4 free generals.

Free generals

To summarize the free generals in the first three theaters:
 - Axis: 1 infantry, 2 armored, 1 navy
 - Allies: 2 armored, 1 artillery, 1 navy, 1 air


Axis generals
The most important type of general the axis lacks is an artillery general. The axis have two noteworthy artillery generals: Leeb and Badoglio.
 - Leeb costs 795 medals and has 4 medals in artillery, but only 1 in movement.
   - Skils: Fire Focus (+12 damage on artillery), Artillery Leader (12% chance to attack again on artillery) and Accuracy (+24% critical hit rate on artillery).
 - Badoglio is a bit cheaper at 640 medals, but has 3 medals in artillery and 2 in movement.
   - Skills: Economics (+6 economy output), Fire Focus and Accuracy.

For a difference of 155 medals, you are trading one medal for artillery in for one movement and Artillery Leader for Economics. Furthermore, Badoglio is only in the lineup of the first theater, while Leeb is in the lineup of the first two theaters. 


The free Axis tank generals only have 2 medals in armored, so getting a strong amored general is also worthwhile. Despite quite a large number of axis armored generals, the two strongest ones are Guderian and Rommel (obviously).
 - Guderian costs a total of 1020 medals and hes 4 medals in armored and 3 in movement.
   - Skills: Blitzkrieg (50% chance that the enemy won't retaliate), Anti-Armor (+24% damage to armored units) and Tide of Iron (+12 damage when commanding armored units).
 - Rommel costs 995, he also has 4 medals in armored and 3 in movement, while also having 3 medals in artillery (if you desperatly need an artillery general).
   - Skills: Desert Fighting (+12 damage in deserts), Tide of Iron and Panzer Leader (+24% chance to attack again).

Rommel is only 25 medals cheaper then Guderian, but deals extra damage in deserts (very useful for the 1941 Northern African theater) and Rommel can function well on artillery, if you are in Dire Straits. But Panzer Leader is a bot over-the-top as tanks already have a chance to attack again after killing a unit, so this skill is only useful against strong enemies or when commanding armored cars. While Guderian's Blitzkrieg skill is always good and Anti-Armor makes Guderian a tank-killer. Furthermore Guderian is in the lineup in the first two theaters and Rommel will deffinetly be in the lineup of the 1941 Northern African theater and hopefully the 1944 Western Front theather (Although he only held his command for the first 4 months after the allied invasion).

Allied Generals
Since you can no longer build units in cities and factories, like you used to, but instead need to rely on the given units and those you can get from reinforcements, you need to rely a lot more on infantry units. And because of special forces, among other factors, they are actually quite strong. Getting a strong and reliable infantry general is deffinetly worthwhile. However you are not given a free infantry general to fight in allied campaigns.

The strongest allied infantry generals are MacArthur, Vasilevsky and Konev.
 - MacArthur has 4 medals in infantry and 3 in movement and costs between 920 and 995 medals.
   - Skills: Fire Suppresion (Reduce 40% damage from counterattacks), Raider (+24 critical hit rate) and Infantry Leader (24% chance to attack again).
 - Vasilevsky costs 750 medals and has 4 medals in infantry and 2 in movement.
   - Skills: Street Fighting (+5 damage in building areas), Crowd Tactics (+10 damage when own army size is larger then that of the enemy force) and Ace Forces (+80% XP of his units).
 - And last but not least (but sadly demoted to 3 star general), you've got Konev at 710 medals with 3 medals in infantry and artillery and 1 in movement.
   - Skills: Crossfire (+12 danage when counterattacking), Encouraged (extra recover morale after annihilating the enemy) and Snow Fighting (+12 damage in snow areas).

Konev is the cheapest of the threeand is an infantry/artillery general, but has a lower base damage on infantry. However MacArthur has the best skills for infantry, but unlocks in the third theater (1942 Pacific War). Because of the mass assault tactic the Soviets used, having an infantry unit is very important in the 1941 Eastern Front theater. So it might be worthwhile to get Konev or Vasilevsky first. Konev only costs 40 medals less than Vasilevsky but has more base damage on infantry, while KOnev can also be used as artillery. Neither of them have infantry specific skills, but decent skills nonetheless. Vasilevsky's Crowd Tactics increases his damage when he controlls an army larger then the enemy (attacking a single stacked tank with a double stacked unit or more activates this skill), and his Ace Forces skill allows his units to increase level faster and thus increasing its hp and damage early on. His street fighting is also very useful in Army Group mode where you will find cities or towns in many tiles. While Konev deals extra damage in snow areas, making him useful in the 1941 Eastern Front theater in both campaign and army group. It is now also way easier to reduce the moral of a unit to 0, so because of encourage, Konev will probably never encounter this issue. Crossfire is also a very usefull skill.
Further more Konev and Vasilevsky are only in the lineup of 1941 Eastern Front, while MacArthur will probably be in 1942 and 1945 Pacific war.


Summary
You do not need generals to complete 1939 Western Front. However getting Leeb as early on as possible might be worthwhile. You'll need an artillery general for the axis anyway and he will grant you bonus medals if you use him in 1939 Western Front and 1941 Eastern Front. If you don't want Leeb, or you want someone cheaper, you can get Badoglio, who'll get you bonus medals only in 1939 Western Front. Though you can skip this if you have bought Zhukov, but having two artillery generals is not a bad idea.
Infantry has become a very important unit in GOG3, but the allies lack a free infantry general. Once you reach 1942 Pacific Front, you can purchase MacArthur, who is, in my opinion, by far the strongest of the choices. If you need one before that, which you will probably need in 1941 Eastern Front, Vasilevsky or Konev is your best bet, although they are probably an even pick.
The axis doesn't have a proper tank general (both only have 2 medals in armored), so Rommel or Guderian are your best bet. They are pretty much equal, so you have to choose based on your situation, Guderian might be slightly better in overall situations than Rommel, but Rommel is a lot stronger in the desert and still strong in other situations. However if you have bought Manstein, you can delay this and go for another general you might need more, before coming back to this choice.

Note 1: At point of writing this, I haven't bought a general and am only at 1941 Eastern Front.
Note 2: I'll probably improve/extend this, once I progress further.