I'm going to recommend the Muv Luv series of visual novels. This is a military sci-fi story where the lead of a slice-of-life harem/romcom finds himself in a war-torn alternate timeline, and has to learn to pilot giant robots in order to fight alongside battle-hardened versions of the girls from his own world. There are 3 parts: Muv Luv (romcom), Muv Luv Unlimited (darker military focus), and Muv Luv Alternative (very dark military/war). Muv Luv Alternative is the #1 ranked VN on vndb.org, with an average score of 9.28. Apparently it's also the highest ranked on the equivalent Japanese site.

The series has many virtues, but also some major caveats.

I'll start with the good stuff:

• Amazing battle sequences - the animation is very advanced for a VN and the best battles are among some of the most tense and exciting I've experienced in any medium.
• Interesting exploration of duty and identity in the politics arc.
• Fun character interactions and humor.
• Many very emotionally compelling moments - this story made me cry more than any other I've experienced (for both happy and sad reasons).
• Pretty smart writing in general - characters don't obviously hold the idiot ball unless they're supposed to be idiots or are acting emotionally at the time.
• Great world-building with well thought-through military/technical elements.
• Great character development for the main character.
• Amazing, film-quality music (more so in Alternative).

Downsides:

• Extremely long overall - at least about 80 hours for the whole thing.
• The first part ("Muv Luv") is a very silly and fluffy romcom, and while it's often amusing, it's over-long and some parts really drag (ugh, the lacrosse). By contrast to Alternative's 9.28 average score and #1 ranking, this only got 7.14 and #476, and shouldn't really be skipped due to important character and plot setup.
• Only available in English via an old pirated version that requires some technical fiddling to get it to work (comes with a readme to explain this).
• Has gratuitous sexual/fanservice elements.
• Some of the tear-jerker moments may come across as heavy-handed or manipulative if you're not in the right mood for them.

Neutral elements:

• Not heavily decision driven - the main routes in Muv Luv are extremely obvious, and Alternative only has one route.
• The prose (and/or translation thereof) is quite simple and straightfoward, compared to eg Fate/stay night.

I wasn't sure whether to post about this because of the downsides, but I read it a couple of months ago and it's still on my mind enough to feel worth recommending. I'd strongly recommend it if you have the time to spare and like both military sci-fi and emotion-heavy stories.

Content warning: Explicit sex and violence.