(NO SPOILERS. Full review after watching the entire show in 4 months. Sorry if it's too vague, tried to stay away from spoilers as much as I could)
So season 1 is pretty good. I really liked the idea of 100 kids being sent back to earth to see if it's survivable. It hooked me just because of all the possibilities. Also the events that take place after they get to earth make you want to keep watching.
Season 2 I think was my favorite. There's a great ending to season 1 that sets up season 2 (Couldn't imagine having to patiently wait for the next season back in 2014 with that ending). At first you're not really sure what direction the story is gonna go but once you find out it's hard to stop watching.
Season 3 goes into an unexpected but interesting direction. Opens up the story to some more lore and backstory which I appreciated because it kind of answers questions about how and why the state of world is where it's at. I liked the season but I can see some people getting thrown off just because how different it is compared to the previous 2 seasons.
Season 4 is still in the direction season 3 took the story just because of how season 3 ended. It's good in my opinion. The conflicts that all the characters go through make it an entertaining season.
Season 5 is decent. It goes into a completely different direction than the previous 2 seasons. I'd say the main issue/storyline isn't as interesting as any of the previous 4 seasons but still is fun to watch.
Season 6 is where the story really just goes off on its own. Much more than any of the other previous seasons. After reading a lot of the other reviews it seems that at this point is where some people really did not like the direction the story went. Don't let the reviews scare you though. I think season 6 can still be enjoyable to watch depending on who you are. More of a hit or miss season though more so than the other seasons. Personally I liked it but I can understand people's frustrations and criticisms about how they managed the story at this point. It's a weird but cool creative twist in my opinion and makes for an interesting season.
Now season 7. It's an interesting season to say the least lol. I don't hate the season but I also don't love it. There's definitely some flaws with it, one that comes to mind is the pacing. There's just so much they try to go over and so many different storylines they need to cover that it gets overwhelming and a little confusing. Without spoiling anything, there's a couple episodes where you're following a singular storyline for the whole episode and just completely pause the other storylines. There's also some questionable decisions made with some of the characters too. I personally think season 7 starts out pretty solid the first few episodes then once they start introducing a bunch of new things and splitting up the story into a bunch of different storylines is where it throws off the pacing and gets confusing. There was still some good moments in the season though and since you've spent all this time with these characters over the last 6 seasons you're still gonna at least enjoy watching them for one final season. The season isn't terrible and is still overall enjoyable to watch in my opinion. The ending you might hate it or love it I have no idea. I just can guarantee you won't see it coming. I personally thought how they ended it was a bit weird. Like it was just confusing and not what I was expecting, also them tossing in so much new stuff without going into much depth about all of it didn't help. Without spoiling, the ending at least gives the viewers some relief and finally puts an end to the long 7 season story.