Flight of the Migrator - The most consistently heavy Ayreon album, this one comes close at times to just being straight-up power metal. A different lead vocalist for each song, but each song has only one lead singer (plus a background singer in one case). This one has an A+ Bruce Dickinson performance on Into the Black Hole, which, for a Maiden fan, is worth listening to for Bruce even if you pick a different album. Also the only one of these that is not a double album.
It is a single album, but I will add, that if anyone new is trying to follow the narrative at all, it's a direct sequel to The Dream Sequencer, both albums being part of an intended double album. But if you're going in strictly for the music, then the lyrics' meaning won't matter much anyway. It is a very heavy album, though, so if you want a more power metal feel, go for it, but it's really just one side/aspect of Ayreon's music as a whole.
The Source - Probably the second-heaviest Ayreon album after FotM. Like 01, has a lot of different vocalists with a lot of switching. IMO this is a slightly stronger cast of singers, but there's some overlap and opinions may vary. This album had a bit more of a mixed reception, but it's personally my favorite Ayreon album.
I wouldn't say the reception was mixed, as far as critics go. According to its wikipedia article, a lot of reviews gave it at least 3.5/5.0 or 9/10. But looking at ProgArchives, it is the fifth highest rated Ayreon album (behind The Human Equation, Into The Electric Castle, The Theory Of Everything, and 01011001 in that order), although it only receved 255 ratings (compared to 600-1200 ratings for the other four albums).
If you want to check any of these out on YouTube, there's a great account named Taycott who has uploaded lyric videos for whole albums (some of which do not have the audio due to copyright claims by the label), which feature pictures for each character and on-screen lyrics by each of them as they sing, so if you want to keep track of who is singing when, it's definitely worth checking out. They also put up a motion comic version of the comic that came with Transitus, though I haven't checked it out in its entirety.
Are those the only 5 Ayreon albums?
There are ten Ayreon albums (nine if you count the two Universal Migrator albums as one double album, though both were released separately).
1995 - The Final Experiment (71:10)
1996 - Actual Fantasy (re-released with a new mix/real drums as Actual Fantasy Revisited in 2004) (61:59)
1998 - Into The Electric Castle (104:32)
2000 - Universal Migrator, Part 1: The Dream Sequencer (70:09)
2000 - Universal Migrator, Part 2: Flight Of The Migrator (65:34)
2004 - The Human Equation (102:14)
2008 - 01011001 (102:09)
2013 - The Theory Of Everything (89:33)
2017 - The Source (88:31)
2020 - Transitus (80:41)
That is the core Ayreon discography, not counting singles, compilations, or live albums. The last five albums are all double albums, as is ITEC.
EDIT - Added album lengths for each album.
-Marc.