I'd say that it depends on who your favourite characters are and which geographical region you like the most. My faves are in 3rd, and I like Crossbell from a technological standpoint. I like how battles flow in Sen no Kiseki, too. All of the games have their merits, so it's hard to recommend which ones to play and which ones not to play. For the most part, though, they're very consistent games. Even Sen, but it depends on how much you like the school setting and the one tsundere character who accidentally gets her boobs touched.
Think of Kiseki like Suikoden. They can all be played in isolation (save for maybe the first two games), and they have their own stories. Most of them are reasonably good and they do worldbuilding very well, trying to make players want to get to what is the "Harmonia" of the series in the form of Erebonia.
So it's like...
Legend of Heroes 6 / Sora no Kiseki / Liberl games = Sora no Kiseki FC, Sora no Kiseki SC, Sora no Kiseki the 3rd
Legend of Heroes 7 / Crossbell games = Zero no Kiseki / Ao no Kiseki
Sen no Kiseki and Sen no Kiseki II don't have a LoH number, I don't think. And they both should take place in the same geographical area (Erebonia).
Regarding LoH6/Sora no Kiseki, the first game basically sets everything up for the rest of the series, and the second game has a lot more action and stuff going on than the first. I like 3rd the best because you get a ton of character development and it focuses on my favourite character in all of Kiseki. But 3rd is nothing without the first two, so you do need to play it after the first two. A lot of the characters introduced in the second game get more development in the third game, so if there were people you liked in the second game, it's best to play the third game also. It acts as a decent bridge between Sora no Kiseki and the Crossbell games and you get to know more of the characters in-depth if you do their character quests.
I'm still in the middle of the second Crossbell/LoH7 game (Ao no Kiseki), and I think they're okay. The technology in the Crossbell area is more advanced than the area in the first three games. There are some cameos from characters in the first three games, so you get more out of it if you did play the first three games before LoH 7. The music in the Crossbell games is really cool, so I always figured I'd like them the most, but that isn't the case. As I said, I'm still playing Ao no Kiseki and I got far enough that I like it a bit better than Zero no Kiseki.
Sen no Kiseki takes place at the same time as Ao and Zero, but elsewhere. So you get to see both sides of the mirror, so to speak.
I'd say try to play them in order because you end up getting -everything-, from the character cameos to all of the geopolitical stuff if you're playing it chronologically. Think of it as progression from Suikoden 1 -> Suikoden 2 -> Suikoden 3. You get a little more out of the first three Suikoden games if you play them in order, even if you can play them in isolation.
I think the series as a whole is good. I didn't like Sen as much because of a few characters and parts of the school setting, but the last few chapters are pretty decent. Still looking forward to getting Sen 2.
As for Nayuta no Kiseki, that game feels like a cool blend of Ys and the Kiseki games (ie: the story/script feels Kiseki-like, but the game plays a lot like an Ys game albeit easier). You can play it in isolation if you wish since it isn't linked to anything. It's one of my fave PSP games.
Also going back to Akiba's Trip, this review is completely brutal.
http://www.gamekult.com/jeux/test-akibas-trip-undead-and-undressed-SU3050175268t.htmlAkiba's Trip 2, c'est donc ça : un enchaînement de quêtes sans intérêt, dans un décorum surlaid mais fidèle au quartier d'Akihabara, avec des combats moisis mais une idée marrante dans le fond. Un jeu où votre petite soeur hikikomori vous fabrique des items nuls et interchangeables, pour le simple plaisir de frapper des pseudo-racailles à duvet en brandissant un sabre kebab à deux pas du Club Sega. Indigne d'un jeu PS4, ce qui n'a finalement rien de déshonorant quand on vit avec les gênes d'un jeu PS2, le jeu ne survit pas bien longtemps à la lumière du soleil. Ce qui s'appelle se faire désaper le moral.
Basically the quests feel worthless, the game's ugly, and the battles kinda suck ("but it's a faithful representation of Akihabara"). The concept on paper is funny, but it isn't executed very well. It also doesn't feel like a PS4 game and seems stuck with PS2 limitations.
The review also goes on to say that the loading screens happen like every 20 steps to the point where you don't want to explore anymore and just fast-travel instead, and that battles are incredibly repetitive.
