DISCLAIMER: First of all lets state one thing, I will be comparing the generations of the pokémon games and I have played on all generations at one point but more so from the third generation and up (this being third generation, but I think you might already know that) so comparisons of first and second generations might be a little bit vague. End of disclaimer.
As you can guess form the title I believe that Pokémon Emerald is the best pokémon game. However you might be thinking why, because surely a series will get better as it goes on, but I think it got it right from classic pokémon to new innovative parts of game play. For example I thought 5th generation took all of the classic parts of previous titles (like Red) and threw them away, and the story from the 4th generation  was just poor.
Any way back to Emerald. This game is incredibly addictive and even deserves a ten for that one, but because of this I do not recommend this to be played any time in-between times when you wish to be doing some-thing else , such as going some-where or just going to bed if you need to get up early for any sort of reason. This is because (fun anecdote time) I woke up one morning at the weekend and realised I had nothing to do. Then, remembering this site, I thought I'd go on pokémon emerald for an hour or two... several hours later, well you get the point about that. Even though the graphics and animations are not up to standard compared to that of Black 2 or White 2 but compared to the entire game boy advance archive of games it comes quite close to top on the list of animation and graphics, and they have aged very well compared to Red and Gold (not Heart Gold, the original). The animations for certain moves such as tail whip can get incredibly tiresome with it's overuse even late in game but these animations can be turned off in the menu. It can be moderately difficult if you aren't willing to add 5 to 10 minutes for each pokémon after end of each gym if it was a tight squeeze which can lead  into a vicious cycle but this is unlikely as it is an enjoyable processes unlike in other games. Just seeing that level up part of the screen is just as exiting when not expecting it, to knowing it will happen, from early on and on until you reach the level cap of level 100 however unlikely you will get there because the slogan of "you gotta catch 'em all" is still in force in Emerald so you are not likely to have the same six (or less) pokémon in your team from your first gym  to the Elite 4 however it is more likely that this will be the case as you get access to the better pokémon as you go on through Hoen (not sure if I spelled that right) such as going from weak pokémon like zigzagoon to sable-eye (which to this day has the top type resistance count only matched by spiritoom or others in the later generations with the same dark ghost combinations) and rayquazza at the end as the legendary. But enough of you reading just go play this fantastic game.

i loved this game so much. but in my opinion pokemon blue is better and it is really fun.