Must-have overseas holdings?

Breuni, atjeh, eygpt Libya and Ethiopia are all must haves before 1870, Ethiopia is surrounded by high life rating areas which are colonizible by state and Goverment, assuming your playing hod, if not Nationalism and imperialism. If ur playing HOD you can do a LOT of colonisation building naval bases EVERYWHERE especially at home, and building a lot of ironclads in home ports and monikers in colonies for maximum colonial points. Latter on areas include dutch Indoneasian areas such as Sumatra, Dutch Borneo and Java aswell as Abu Dubai. Sudan, which is apart of eygpt, has rubber as well as the Congo and Central Africa, Sumatra, Java, maylasia, uncolonised Borneo and atjeh, my personal favorite to take is from the UK Ceylon just off of India, Vietnam and Cambodia and Siam all of good rubber . Breuni has oil so does Abu Dubai, fez, in morrocco, East and west galcia in austria, un colonised Borneo and North Borneo, Nejd, part of hail and hedjaz, Muscat in Oman, so take one of thier regions and establish a protectorate against the rest. Sumatra also has oil and sulawasi- the island that looks like a K in indoneasia. As well as an area south of Algeria and in tripoli and of course california and Texas and Albania + Romania, Southern perisa and the caucuses and Iraq. hope this helps.

Oil is very late game, so I'd consider it a not so important resource, especially since, as Germany, it's easy to sphere Netherlands and whoever controls the oil producing provinces of Austria (or just conquer it during a great war).
Direct control is better than sphering because you have better tech and railroads, which means you'll produce much more from the same province a,d it helps a lot with goods that are in constant shortages as rubber and coal (because if you keep industrializing as Germany you'll have to import coal)

Early conquests of uncivs are also useful to get navalbases and be in range of uncolonized provinces. As Germany you can be a main colonizer of Africa, you just need to update all your bases on your coast and build a nice fleet of ironclads. Then one base in Morocco, one in Zulu, one in Indonesia and you should be able to access most of the colonizable land. By the way, colonizing easter new guinea leads to a small specific german decision.
You also have to realize that colonization can be worth it just to deny it to your competitors.

In Egypt you'll find cotton and latter rubber in the south, Sinaï is nice to have for the canal, Eritrea can be a good naval base. Southern Morocco and Zulu only if you want the naval range to compete in mass colonization of Africa. In Asia, Atjeh for a port, tea and rubber. Then Suzhou for tea and silk, or Taiwan if you're lazier. Going full China is very powerful but can feel a bit gamey.
Korea and Manchuria are popular targets for their coal and iron which is not an issue for you, now, but it could become one in the 20th century, even for sphering purposes it can be better to have a foot there.

Concerning rubber, you'll have some in Cambodia, southern Thaïland, Ceylon, the southwesternmost indian state.