I've never played a Pokémon game ever since Red version back when I was in high school, however a friend of mine is selling me his copy very cheap so I started to think that maybe I could give the series another try. When talking about good entry I don't mean in terms of learning curve or accessibility, as these games tend to be fairly easy, but in terms of content & spirit of the series. 

@iGaboru: Is it Diamond, Pearl, or Platinum?  Edit: In any case, the revised edition, Platinum, is "better." Having played through all of the games, I'd honestly only recommend Yellow, Crystal, and Black or White at this point in time. 

I tried playing through it and you get to a point where you have to find every single pokemon in your pokedex to continue the story. That's what I'm getting from it at least. I have tried to play Diamond for years and I have no idea where I'm going after I beat a certain gym leader because they won't let me pass through a certain gate. I played Yellow when I was a kid and loved it though.

I'd say start at Heartgold if you held out this long. 
Diamond and Pearl is kind of when a large number of people decided "fuck pokemon". On the other hand, Probopass.  

sure.
 
Pokemon games haven't changed much, and personally, I've never found them to be difficult.

I always have, and will continue to deem Generation 4 (i.e. Diamond/Pearl/Platinum) as the worst of the Pokemon games. That doesn't make it a bad game, it's just not Pokemon-good in my opinion. That said, Pokemon Emerald is probably my favorite of the series, and should be pretty damn cheap, since it's a GBA game.

Hmm, interesting comments. I'm gonna have to reject his offer then :(