It's a good series. Regarding which is the best,
Hobo Elf is right that there is no real consensus amongst the fandom as to which is the best, and the series has overall been very consistent in quality.
IV usually gets derided as the easiest, but I'm not so sure this is entirely fair. I mean, on the one hand, the dungeons are less complex, but on the other, the game has much better combat balance than many of the others, especially III which was very easy to break by subclassing. Also, it is not like the dungeons in this series are ever as crazy as something like, say, Elminage Gothic, so to say that the games are harder because of the dungeons is a bit misleading. In fact, most of the dificulty of these games comes from the combat, even the dungeon design itself relies a lot on tough combat in the form of FOEs to achieve challenge, so the differences in dungeon design don't really make that much of a difference. It should be noted that it is moot anyway, as all of these games are harder than most Western RPGs, and they even beat some of the serious Japanese crawlers in this regard, like Stranger of Sword City and the Operation series.
This brings us to how the series compares to other Japanese dungeon crawlers. As I said, it beats many of them in terms of challenge, but overall it sits around the middle, somewhere in between the harder ones like Elminage Gothic and Dungeon Travelers 2, on the one hand, and the easier ones like Stranger of Sword City and the Operation series, on the other. That said, I wouldn't say they are necessarily better than SOSC, as the latter has some things over them like more varied dungeons, better itemization and an interesting permadeath mechanic. I also prefer the more DnDish integrated character system centered on multiclassing used by SOSC and Elminage over the granular system of EO which revolves around the "perks" or skills that you build your characters around. That said, both EO and Dungeon Travelers 2 execute these granular systems very well, so party and character customization are still great in both of them, plus they both complement that with interesting subclassing/class advancement systems. I would say, though, that the Grimoire system used in the EO Untold series is kind of lame.
Itemization is not terribly exciting, but it is not downright obnoxious like in, say, Divinity: Original Sin 2. The combat is better than in most dungeon crawlers except Labyrinth of Touhou 2 and Dungeon Travelers 2. Dungeons are generally among the more complex, but as I said, they don't really touch the cream of crop of the genre.
Overall, I would say the better EO games are not as good as Elminage Gothic, Dungeon Travelers 2, and maybe also Elminage Original and Stranger of Sword City, but they are still great in their own right, and beat most of their other contemporaries in the genre. IV is probably the best entry point as it is the most accessible of the series, but don't be discouraged by the talk of it being "casualized", it's still very much a serious EO game and worth playing.