"I love Germany."
Short version: (大)好き is always used with が.
Long version: The difference between は and が is immediacy.
Both can be markers for a subject, but it is more to correct to describe their function as 'marking the main topic of the sentence'.
Froma usage standpoint, using は twice would just be confusing and/or in bad style.
From a grammatical standpoint, in sentences talking about emotions が marks a direct object as often as it marks a subject, because in the end the grammatical object is usually what the sentence is about.
Example: かれはとりにくがきらいです。[He hates chicken.]
The boy is the grammatical subject, but what we are talking (and having strong feelings) about is the chicken. So the word/topic most immediate to the actual emotion (and most important to the meaning of the sentence) gets the marker が. The less immediate word/topic gets は. If you leave out the person (which you always can in Japanese), then you get とりにくがきらいです. The literal translation is "The chicken is hated/disliked." But everyone who hears or reads the sentence knows, that it's ultimately hated by someone. So you just go ahead and use が, because everyone is adding the less immediate topic (the person having feelings) in their head anyway.
IMPORTANT (Grammar): The は-topic always comes before the が-topic, never the other way around!
IMPORTANT (Usage): In a sentence with two topics (one using は, one using が), you can leave out the part of the sentence that cointains the は-topic without changing the core meaning of the sentence. But never the otherway around!
かれはとりにくがきらいです [He hates chicken.]
とりにくがきらいです [The chicken is hated (by someone).]
かれはきらいです [He is hated.]
For the actual example above, ドイツは大好きです should technically be accepted, as it would mean "Germany is loved". But in natural Japanese it is generally assumed that you talk about your own feelings, whenever you talk about an emotion. So the implied sentence here (and what a Japanese person automatically hears) is:
わたしはドイツが大好きです. [I love Germany.]
So if you need to translate "I love Germany", then you HAVE TO use が.