While Rambam (Melachim 5:12) writes that it is forbidden to leave Eretz Yisrael, nonetheless he does not include it in his list of 613 Mitzvos. Some explain that Rambam only holds that this Mitzva is Derabanan, while others (Megilas Esther) hold that this Mitzva applied specifically when Yehoshua conquered the land.
Ramban (Hasagos HaRamban 4, Bemidbar 35:53) disagrees with Rambam, and writes that certainly living in Eretz Yisrael is a Mitzva today. The Pischei Teshuva (EH 75:6) paskens like Ramban, and quoting the Sifrei (Re’eh 28), writes that the Mitzva to move to Eretz Yisrael is equal to all other Mitzvos. (See Igros Chazon Ish, 1:175 and Yechave Daas 4:49)
R’ Moshe Feinstein (Igros Moshe EH 1:102) wrote that while most authorities agree with Ramban, it is a Mitzva Kiyumis rather than a Mitzva Chiyuvis, optional (like tzitzis) rather than obligatory. While Rambam writes that one mustn’t leave Israel, he doesn’t write that one must move there. R’ Moshe quotes Rabbenu Chaim Cohen (Tosafos Kesubos 110b) who defended the practice of those who didn’t make aliya, as moving to Israel was considered dangerous.
Amazingly, the Pischei Teshuva quotes the Maharit (Shut 28) who writes that this opinion was mistakenly inserted into Tosafos by a student. Since when did a Mitzva stop being applicable simply because it is difficult to observe?!
R' Avraham Borenstein of Sochachov (Avnei Nezer 2:454) writes that R’ Chaim Cohen’s reasoning is no longer applicable. In spite of the potential difficulties, this is a Mitzva like no other. Furthermore, living in Eretz Yisrael enables one to fulfil so many other Mitzvos. (See Tzitz Eliezer 7:48:12)While there are factors that may prevent one from making Aliya, one certainly fulfils a Mitzva (according to most Poskim) in doing so. We must bear in mind, too, the words of the Chasam Sofer (Derashos 1p18), The ideal way to observe Torah and Mitzvos is specifically in the land of Israel.