It can be all of the above.
- access/ownership to natural resources off the coast
- possession of Sevastopol, which carries massive emotional and historical weight to Russians, not to mention is a significant Naval port.
- possession of Crimea itself, which has been largely Russian since Stalin forcibly exported the remaining Crimean Tartars to Siberia and started bringing in Russian citizens. From 1991 to 2014, Crimea was one of the highest Oblasts in terms of percentage of votes cast for Russian friendly politicians
- because they could. The Ukraine government was in turmoil, with no one really knowing who should be in charge after Yanukovych fled. Many of the top admiralty were known to be loyal to Russia and would switch sides immediately given the opportunity. In particular, Sergei Yeliseyev, the acting commander of the entire Ukraine Navy, who is now Deputy Chief of the Russian Baltic Fleet, and Denis Berezovsky, who was appointed commander in chief after Yeliseyev defected, also defected, and is now the deputy commander of the Russian Black Sea Fleet. The takeover of Ukraine Navy assets in the Crimean operation was largely an inside job.
Trying to pin the takeover of Crimea on any one of those bullet points is missing the bigger picture.