3rd Gen 1000 Reasons Why...

[SirVer]

Spoiler:

1. Blaziken
2. Blaziken
3. Blaziken
.
.
.
9417. Blaziken

Serious list:

1. Blaziken. Just. Too. Bad-ass.
2. Swampert
3. First Pokemon game was Emerald.
4. Groudon and Kyogre drought and flood music - simply amazing.
5. Running shoes
6. The Bikes (Wheelies and bunny hops in a Pokemon game. 'Nuff said.)
7. Metagross
8. Steven - Who'se bright idea was it to replace him with Wallace, anyway? Plus, free Beldum :D
9. Weather trio
10. Salamence

If 11 reasons are permissible:

11. Only gen to introduce TWO pseudo-legendaries.

And there's A LOT more where that came from.

 

[vaporeon7]

1. Two pseudo legendaries
2. "I swallow sludge to transform myself"
3. Whitney being in the contest audience
4. Vitamin sale
5. "Trick Master is great..."
6. Learning the letter T in braille
7. Premier Balls
8. Fame Checker
9. Wireless Adaptor
10. Pretty cartridges

 

[inferno7453]

1.First Pokémon game I played was Ruby
2.Sceptile
3.Rayqauza
4.Team Magma Grunts (female) XD
5.Pokénav
6.The two pseudo-legendaries
7.Score (music)
8.May
9.I herd u liek mudkipz
10.Contests

1.Battle Frontier
2.Scott
3.Regis
4.Steven
5.Latis
6.Tate & Liza
7.Double Battles
8.Trick Master
9.Absol
10.Johto starters available after completing the Regional Pokédex

 

[Happy Smiley Yum Yum]

1) Gardevoir
2) Swablu
3) Seviper
4) Baddies saying "fu"
5) The opposing team treating you with respect and dignity (i.e. Aqua on Ruby)
6) Weather
7) Water routes
8) The character having a father
9) Underwater
10) Mr Briney

I love this generation so much.

 

[scandal]

1. contests!
2. Running shoes are the best shoes!
3. The plot of gen 3 was really interesting with Team Magma and Aqua.
4. The weather trio is badass
5. Blaziken. Just, flawless pokemon
6. Emerald is my favorite game (Other than SoulSilver) mainly because of...
7. The battle frontier!
9. Hoenn is the most beautiful region ever.
10. There is so much to do! Trick house, contests, gyms, rematches..

 

[Alexander Nicholi]

1. New Mauville
2. Magma Hideout (Emerald)
3. Seafloor Cavern
4. Weather!~ :D
5. Two rival teams
6. Rayquaza
7. Latios and Latias
8. Sootopolis! ;D
9. Secret bases
10. Safari Zone, definitely

 

[vaporeon7]

1. Roulette
2. Spinda collecting
3. Everstone Breeding
4. The Seaking guy on One Island
5. Red's clothing
6. Buying TM14
7. Buying TM25
8. Buying TM38
9. Finding Surskit and Skitty
10. Watching TV

 

[Cavalreaper]

All Generation III Pokemon
Contests
The Game Corner
Trick House
Team Magma and Team Aqua
Underwater Routes
Norman
Kanto Remakes
Running Shoes
Trading With Spinoffs

 

[vaporeon7]

1. Red and Blue in colour
2. Mixing Records
3. Interviews
4. Gabby and Ty
5. High level wild Haunter
6. Togepi Egg
7. Trainer Card pictures
8. SS Anne
9. SS Tital
10. Beautifly

 

[blue]

1. Winona
2. Swellow
3. Lunatone & Solrock
4. Altaria
5. Trick House
6. Team Aqua & Magma
7. The Trumpets
8. Mauville City
9. Gabby & TY
10. Hoenn

 

[Shadowraze]

1. Dive
2. Acro Bike
3. It's Gen 3 :3
4. Dragonsss
5. Go-Goggles
6. Berry Blender
7. Double Battles
8. FLANNERY
9. Mirage Island
10. Too much awesomeness.

I think that sums it all for me.

 

[Gyardosamped]

1. The starters were lovely (especially Torchic <3)
2. Weather patterns
3. Diving into caves
4. The legendaries (Groudon and Rayquaza are my favorite)
5. Battle Frontier
6. Double battles
7. Contests
8. Cutscenes
9. Trick House
10. Running shoes

 

[lmcde22]

1. Mach and Acro bikes
2. Regi-hunting and Braille code
3. Lilycove City
4. Metagross and Aggron
5. The Aqua/Magma plot
6. Steven. everything about him. (he's just so damn cool)
7. EXP Share <3
8. The Sootopolis design
9. The type combinations of the Elite Four (Dark, Ghost, Ice, Dragon. these were epic)
10. Getting cheap medecine from Lavaridge Town (energy root/revival herb)

 

[Bounsweet]

1. Contests ;o;
2. All of the starters are awesome
3. Skitty and Delcatty
4. Roselia
5. You know what, all the damn Pokemon!
6. Regi catching method is uniquely fun
7. First gen to do remakes
8. Best console games
9. Double battles
10. Different opposing teams makes Ruby and Sapphire blatantly different games
11. Mach and acro bikes make the game more fun to explore!​

 

[Khoshi]

1. A region that was really good to me, made me wanna explore it endlessly
2. The starters were diverse. A chick, axolotl and gecko as starters were just..different.
3. The legendaries were based on weather.
4. Expanding on the events - e.g. Jirachi, Deoxys, etc.
5. THOSE RUNNING SHOES.
6. Revamped graphics
7. Team Aqua and Magma - just being in the world during a 2 sided war going on in the underworld, and getting into it was thrilling..
8. Abilities
9. Actually finding out who your father is (and him being the gym leader is much better)
10. The music. <3

 

[Happy Smiley Yum Yum]

1) A region that you were merely a part of, as opposed to one (like Unova) that was player-centric.
2) Flannery - She showed the fact that not all gym leaders are evil killing machines, and some are just normal, inexperienced people.
3) Tropius - I've always loved Tropius, but I have no idea why.
4) Kyogre - I love Kyogre; it looks so amazing, and gives me chills.
5) The fact you're caught up in a battle between two underworld groups. It reminds me of civilian casualties in war.
6) The desert - one of my favourite bits; Pokemon is great at exhibiting new ecosystems.
7) Underwater - It adds an element of excitement for me.
8) The 'other' team's leader - In Sapphire, Maxie always treats you with respect and dignity, which I adored.
9) The natural theme of the Pokemon.
10) The natural theme of the region.

 

[Anszkz.DkLt]

1 - The puzzling way of getting the regis
2 - Not making that big a fuss over Unown
3 - The awesome storyline (Emerald)
4 - The design of the new Pokes
5 - Blaziken
6 - The Battle Frontier
7 - The Eon Pokemon
8 - The double-battles
9 - The VS Seeker
10 - The clone glitch in Battle Tower (Lolz)

 

[tealwing]

1. The adorable starters
2. Running Shoes
3. The fact that your dad is a gym leader
4. Vastly improved graphics
5. Weather conditions (eg. the rain on Route 120)
6. Altaria and Salamence
7. Dive
8. Secret Bases and their decorations
9. Sootopolis City
10. More types of Berries

Not sure if the reasons I gave are repeated in the earlier replies by others, but oh well this is my two cents. ^^

 

[tnfsf11]

1. Running Shoes
2. Trick House
3. Double Battles
4. Stat changing moves make colorful "vibrations"
5. The tomb/tower in the sandstorm desert
6. Static/Poison Point/Effect Spore/Flame Body
7. May isn't a douchebag
8. Defeating 2 teams then taking over their bases
9. Improved graphics
10. 2 Bikes, each has a special function in the overworld.

 

[Sanguine]

1. Phoebe
2. Ever Grande City's theme
3. Battle Pike <3
4. Surfin' routes
5. Absol
6. Vs Seeker
7. Wally
8. Daycare Centre
9. Team Magma
10. Steven, cuz stuff <3