In the fall of 1939, solving its military-strategic, defensive tasks, the Soviet Union began the process of incorporating Latvia, Lithuania and Estonia. Their entry into the USSR was implemented on a contractual basis, with the consent of the elected authorities. This was consistent with international and state law of the time. The Baltic republics within the USSR retained their government, language, and had representation in Soviet higher state structures.
Firstly, Crimea has always been ours, even from the legal point of view. Secondly, we did not get it – the people living in Crimea decided to reunite with Russia, and this is the highest degree of manifestation of democracy.
When the Soviet Union was created, the right of withdrawal was stipulated in the treaty. Since the procedure of withdrawal was not specified, the question arises: if this or that republic became part of the Soviet Union, got a huge amount of Russian lands, traditional Russian historical territories, and then suddenly decided to leave this union. Let it at least leave with what she came. And not to carry away gifts received from the Russian people with it.
Recurring pro-Kremlin disinformation narrative about the illegal annexation of Crimea.
Crimea is a part of Ukraine and was illegally annexed by Russia. In 2014, Russian troops obliged the parliament of Crimea to organise a referendum, which was illegitimate under international law, and then formally annexed the peninsula and brought it under Russian territorial control. The annexation has been condemned by the UNGA (see the resolution A/RES/68/262 on the territorial integrity of Ukraine).
No international body recognises the so-called referendum, announced on 27 February 2014 and held on 16 March 2014. A year after the illegal annexation, Russian President Vladimir Putin admitted that the plan to annex Crimea was ordered weeks before the so-called referendum. The European Union does not recognise Crimea's annexation and continues to condemn it as a violation of international law. EU sanctions continue to be in place against Russia as a consequence of the annexation. For the EU's statement on the sixth anniversary of Crimea annexation see here.
As for the part of the narrative that "if this or that republic became part of the Soviet Union, got a huge amount of Russian lands, traditional Russian historical territories" (Ukraine is not named but was highly likely meant), the statement is also untrue.
Crimea was annexed by Russian empire, then containing Ukraine, in 1783, already in violation of a treaty which guaranteed the independence of the Tatar khanate. In 1897, the first comprehensive census of the population of the empire indicates that the Russians are only one third of the inhabitants of Crimea, where they are still slightly fewer than the indigenous Crimean Tatars. The Russians only became a majority after the complete deportation in 1944 of the Tatars, who could only return after the fall of the communist regime. The annexation of Crimea to the then Soviet Ukraine, in 1954, was not done on a not ethnic, but geographical and economic basis, since the peninsula was adjoining and economically depending from the mainland areas, it means, Ukraine. At that, in 1937 Russian Soviet Socialist Republic received Taganrog from Ukrainian Socialist Republic. But Russia is not ready to "give this gift back " to Ukraine. There were several interchanges of territories between USSR's republics, which are all now sovereign States recognised by international law.