To start off, no, I am not saying which generations are better or worse than others here. That is strictly a matter of opinion and varies greatly between every fan. Instead, I'm going to highlight some of the best things and the worst things from each generation. Because I'm bored.
Generation 1
Pros: First generation of Pokemon ever- it doesn't get much more nostalgic than this!
Cons: Requires link cable for trading. Easy access to game breaking glitches. Psychic type is way overpowered. Internal battery will eventually die.
Generation 2
Pros: Two regions available for your exploring pleasure! Obligatory mention of new features and Pokemon. Can be a huge nostalgia boost for people who played it as a kid. Types are now more balanced. Get to battle Red from RBY. Some of the minigames on the home console games from this time were fun.
Cons: Requires link cable to trade. While glitches are fewer, there are still some bugs. Internal battery will eventually die.
Generation 3
Pros: Obligatory mention of new features and Pokemon. First introduction of wireless trading (short range, but still). Includes remakes of Gen One. Home console games in this generation were superb! Improved graphics.
Cons: Cables still required for a number of games. Many Pokemon were made difficult or near impossible to obtain. Loss of day/night cycles in FRLG/lack of visual day/night cycles in RSE.
Generation 4
Pros: Obligatory mention of new features and Pokemon. Worldwide wireless trading! Vastly improved graphics. Inclusion of Gen 2 remakes. Pokewalker. Comeback to visual day/night cycles! Import Pokemon from your Gen 3 games.
Cons: Loss of Dive HM, lack of good home console games.
Generation 5
Pros: Awesome story, improved graphics, and mention of other improvements. Dive is back! Transfer your Pokemon from Gen 4 over. Worldwide wireless maintained. Pokemon Global Link.
Cons: The new box system (xD), excessive number of Pokemon now needed to "Catch 'em all!".
These are just my quick notes on what I think was good and bad in each generation. Feel free to post your own opinions!!