Read others' reviews first for more in-depth content on individual sites.

There are six temples scattered across the island. Here are my and my wife's experience visiting them in December 2024 during Malta's "off season":

1. Ġgantija on the island of Gozo: Tied for the best of the Megalithic Temples, absolutely worth a visit. A great interior museum area giving you context for the temples, their history, their mystery, and what has been done recently since their re-discovery to preserve them. After the interior museum, a walking path leads you down a beautifully manicured landscape to see the astoundingly-preserved temples and walk inside them.
2. Ħaġar Qim South of the airport: Excellent, just like Ġgantija, but offering a different type of experience. This temple has a great audio guide that you can load onto your phone and it will guide you through the whole experience as you wish. (It's very good, I recommend it.) There's an interior musuem area that is more "interactive" than Ġgantija's and focuses on different aspects of the temples than the other museum does. After the great museum, there is a long outdoor walk to two major temples that are well-preserved and have a lot of neat history to see inside. Hiking trails are also available on the premises to visit other minor landmarks.
3. Mnajdra: This is wrapped up on the same large park site as Ħaġar Qim but technically requires a separate payment. If you're visiting Ħaġar Qim, you should just do Mnajdra, they're practically one and the same visit. Side note: I disagree with others like Nan & Clyde that the tent coverings take away from the ambiance of these sites, I think think the coverings are just fine and undistracting.
4. Ta' Hagrat in the North of the main Malta island: A sharp decline in quality after the previous three temples. This is located at the end of an urban alleyway in a small town (village) on the main Malta island, and is not at all worth visiting. The site has a metal fence surrounding it and looks like it could be a small, forgotton, municipal graveyard. There is technically a security guard, probably the bare minimum required by UNESCO, who sits in a booth inside the site, but who frankly you could probably pass by if you didn't care about being honest. The guard's booth isn't even near the enterance to the small site, it's sitting in the opposite corner. There are two small signs on the ground giving a tiny bit of temple info and everything is overgrown. If you hadn't been to other temples, you wouldn't even know what you were looking at or looking for. We spent roughly five minutes here walking in, seeing everything there was to see in the gated "site", and leaving. Extremely disappointing.
5. Skorba, a five-minute drive from Ta' Hagrat: Somehow even worse than the Ta' Hagrat. I adore history, and even I had to admit that Skorba hardly even rises above the description of "a pile of rocks overgrown with weeds." If the security guard was present in their office, they didn't even come out to interact with us or check our tickets. We spent approximately two minutes walking from the entrance of the gated square to its opposite end, seeing the unmarked deteroriated rocks covered in vegetation, and leaving. I think there might have been one single, tiny, dirty, sign in the site? It's such a shame, because the other temples prove that with the proper outer trappings and care for the sites themselves, these archeological wonders can be fascinating and a great place to experience history coming to life. Skorba and Ta' Hagrat are instead heaps of neglected rocks. There is no reason to go out to visit them other than to "check them off the list."
6. Tarxien: This was actually the only Megalithic site we did not visit. We could have made it work, but we were already opting to visit the Hal Saflieni Hypogeum on the same day and I didn't want to push my wife to visit yet another Megalithic temple after we were already seeing so many on our trip. Other reviewers here say it's good, though, so it sounds worth it. (And definitely visit Tarxien if you have a choice between that and the two Northern sites.)

4 or 4.5 stars for Ġgantija and Ħaġar Qim, but 2 stars for Ta' Hagrat and Skorba. My final rating reflect an average of both of those.