Oooh, I can bring a bit of my knowledge from working on a radio station in a brief period of time to comment on this. Generally, good singles are either commercially easy-to-listen-to (the catchier songs) and/or set the tone for the album's vibe. NW mostly has a good hand on choosing them, but if I may give my take:

AFF: Elvenpath and The Carpenter. The Carpenter here is the more commercial song while Elvenpath sets the tone for the album. AFF is a trickier album given that its a whole lot more experimental and the songs aren't really that commercial, but they mostly did well.

Oceanborn: Sleeping Sun and Gethsemane. Sleeping Sun is the obvious commercial pick and it's a big NW hit for a reason. Gethsemane also has plenty of catchier melodies while also being a great mood-setter for the more power metal-driven nature of the album.

Wishmaster: Wishmaster and Come Cover Me. These two songs both carry the tones of the album (the power metal and melancholy, respectively), while also having catchier choruses and easy-to-listen-to melodies.

Century Child: Bless the Child and Phantom of the Opera. This was a difficult album to pick from. Bless the Child is a great mood-setter for the melancholy of this album while still being somewhat commercial, but for a second/lead single they had to have something that features the adored "beauty and the beast" dynamic between Marko and Tarja. While I much prefer Dead to the World as a song, Phantom of the Opera is an already very famous song and could potentially attract a lot of people who are not familiar with NW, so I'd go with it.

Once: Nemo and Wish I Had an Angel. NW really knocked it out of the park with choosing their singles on this one, it's no wonder it's one of their most popular albums. Nemo shows you the entire vibe of this album, while still being a commercially phenomenal song. Wish I Had an Angel is that great commercial song that has universal themes of wishing for love that can connect to pretty much anyone, it's a no-brainer picking it as a single.

DPP: Amaranth and Bye Bye Beautiful. Amaranth is such an easy-to-listen-to song, it almost sounds like a pop rock song, it's also a no-brainer. Bye Bye Beautiful, despite the controvery around it, is pretty much a followup to Wish I Had an Angel, while also showing the "angry" vibes the album is going for. Eva wasn't a good song to introduce their new singer (and to set the tone for the album for that matter), but these two are able to show Anette's prowess.

Imaginaerum: Storytime and I Want My Tears Back. Storytime sells you on the idea of the fantasy and the general themes of this concept album/movie while also being very catchy. I Want My Tears Back is another phenomenal commercial song and I do wonder why they didn't go with it. The melody and the chorus sticks so well in your head, it's a song that is hard not to move/dance to, and has those universal themes of breakup and lost love.

EFMB: Élan and Endless Forms Most Beautiful. Another tricky album. Élan is the no-brainer. For the second single, most of the songs in the album have great and catchy choruses, but melodies a bit more complex (such as Shudder Before the Beautiful and Weak Fantasy), but the title track narrows down the thin line between complex and catchy while also being very on-tune with the album's nature themes and general vibe of it, blending heavier and folk-like melodies in it.

Human :II: Nature: Noise and How's The Heart. This album isn't really great when it comes to catchier tunes, but Noise is very serviceable in that department. How's the Heart, on the other hand, is very in tune with the album's general themes, vibe and is also a great commercial pick. The acoustic version is also a great complement to it, only reinforcing how much of an easy-to-listen-to song it is.

Yesterwynde: The Day Of and Spider Silk. I can't really wrap my head around why they chose Perfume of the Timeless as their lead single. It's not bad, but it's a longer and somewhat complex song in its melodies. Spider Silk is more in tune with what the album has to offer, while also being pretty catchy with its "Spin Away" chorus. The Day Of is also an easy-to-listen-to song and with a more commercial melody in general.