Red/Blue/Yellow:
- Were the first games (duh)
- Introduced some great, iconic Pokémon and characters
- Yellow had great sprites for the time and did a good job of mixing RB's setting up with anime influences
Gold/Silver/Crystal:
- The very first direct sequels
- Excellent postgame (8 more Badges beats some battle facility ANY day)
- Johto was a fascinating, if small, region
- Lots of great character designs (including Kris, the best player character ever)
- Only Gen to use new Pokémon to accent the older ones and not replace them
- Only Gen to add new features without taking any older ones away
- Crystal still has the best animated sprites
Ruby/Sapphire/Emerald:
- Uhhh... Well, Emerald did a good job of changing things up from RS, and it had the best battle facility, by far
FireRed/LeafGreen:
- Er, they were the first remakes, I guess?
Diamond/Pearl/Platinum:
- Brought back Day/Night and added the P/S Split
- Many of the new Evos were good
- Platinum was the most "complete" Third Version, fixing all of DP's many issues as well as upping the difficulty; it was the first game to truly have a "smart" AI
HeartGold/SoulSilver:
- Were the greatest remakes, willing to change things up (unlike FRLG) without going too far out there (unlike ORAS)
- Gorgeous graphics (compared to DPP and earlier games)
- Best implementation of following Pokémon (compared to Yellow)
- Already great postgame from originals made even better by Gym Leader/E4 rematches and new areas
- The Pokéwalker was neat
Black/White:
- Excellent story
- Did a great job of changing up the formula from the previous games
- Also had gorgeous graphics
- First games to have seasons
Black 2/White 2:
- First mid-Generation sequel
- Still had a good story (was much better at continuity than GSC/HGSS were)
- Graphics still gorgeous
- The PWT was a cool idea, despite the flawed execution
XY:
- First games to not use sprites
- Had many good mechanical improvements over earlier games
OmegaRuby/AlphaSapphire:
- First remakes to dramatically change up certain things from the originals, even if the execution was very flawed