Yeah

Italy & Spain.

Some others do recommend Germany or USSR.

I agree, Germany is easier than Russia.
Just move your troops to the east and try to capture Riga and Leningrad soon so that the Russians don't spamm their generals, than it will be easy.
You can practically steal Krakow and whole yougoslavia, your allies will take care of that. Smigly will also move out of his city, so after taking Leningrad taking Poland is a piece of cake. Then your economy will be strong enough to finish the rest.
If you dont like Germany, then Spain or the US are easy ones.

I think another tank general and then Rommel with 40% discount after that. You need at least 3 tank generals anyway and you already have good generals for Invasions. Another possibility is - to upgrade your current team.

I hired, for example, Leeb long ago, but haven't updated him yet, so maybe it is reasonable to hire new generals only when you have resources for upgrading them. Or else they remain only big names without real skills. And one fully upgraded general is a way better than 3 generals without upgrades.

I think you need only powerful tank generals now. You have Guderian and will have Rommel (after 40% discount). Your third tank general should be also strong enough. All good free tank generals (except Guderian) have 4 or less decent skills basically. So they are more or less equal. I would say Vatutin is a good choice. He is so to speak "Manstein without Blitzkrieg". There are in general 3 objections against him:

1. He has only 4 skills.

But, to be honest, many of his rivals have no more than that actually. Only on paper they have 5 skills, in reality good are no more than 4 of them at best. The only one exception probably is Graziani.

2. He is slow.

I don't think this argument is valid after your technology tree is developed. For example, the difference between Vatutin and Rokossovsky (who has 4 stars) is one hex only. Not a big deal...

3. His rank is low.

But many of his rivals have only 5 tank stars. You have to pay 800 medals more to get the 6th star. You can spare these medals in case of Vatutin and upgrade his rank instead. Besides his Tide of Iron is quite handy in these circumstances.

Absolutely! Generals have to work with synergy like one team. And all positions are important. The main aim of the player - to make all of them be useful in their own particular way...