I've played EO3 and recently started EO2 and I've honestly got to say EO3 is the only one I can recommend.
Unlike, say, The Dark Spire or SMT: Strange Journey, the dungeons in EO aren't very maze-like. The challenge of EO dungeons comes from dodging FOEs. They're monsters represented by pulsing balls of light that are many many levels higher than the other monsters on the floor (and should be many levels higher than your team) so you need to avoid getting into fights with them. Each FOE type moves about the dungeon with a different AI pattern and avoiding them is kind of like
Deadly Rooms of Death lite. Of course, unlike DROD, in EO you're constantly under threat that a random encounter can happen while you're being chased by a FOE, in the FOE's patrol route, etc, and every round of combat the FOE moves one step on the map. If the FOE intersects with your square during that random encounter, they'll join in the fight. That's why the game provides you with a color-coded random encounter chance indicator.
Careful you don't over-level your team or you'll break this key part of EO gameplay by being able to just beat up the FOEs. Mapping out every square of a dungeon alone should produce enough XP to keep a well-balanced team at the sweet spot of difficulty. Also, don't worry about investing points into the cheap skills early on coming back to bite you in the ass when facing the final boss, EO has many options for reshuffling your skill points once earlygame skills become obsolete (the best is retiring).
The worst part of the EO series, IMO, is the farming. If you want to make enough money to keep your equipment up to scratch, either your team has to waste several precious points on farming skills (Take, Chop, Mine) or create a seperate team devoted entirely to farming and avoiding fights to make your money for you.
EO3 is the only one I can recommend because in addition to the FOE dodging, it's the closest any DS game I've played has come to recreating the joy of crafting custom teams in Final Fantasy Tactics using the job system (way more than Final Fantasy Tactics Advance). It has way more colorful and inventive sprite work than EO2. Even the farming is at its least painful because all the key skills for that have been consolidated in the "Farmer" class, and you can give your farming team a skill that lets them collect a fraction of your man team's earned XP while sitting on their butts back home. The main downside of EO3 is the Japanese pro-whaling propaganda lurking behind the seafaring minigame.
In EO2, on the other hand, most of the classes are one-trick ponies and defeating FOEs doesn't even give you XP. If you aren't that concerned with customizing your team, you're better off playing The Dark Spire.
I haven't played EO4, but word on the street is they've dumbed things down.
Two working class dudes, one black one white, just baked a tray of ten cookies together.
An oligarch walks in and grabs nine cookies for himself.
Then he says to the white dude "Watch out for that black dude, he wants a piece of your cookie!"