1. Red/blue/yellow. Original and the best. Loved and completed gen 1 several times over all 3 games. Put in literally 1,000's of hours on all 3.  Also Hitmonlee!
2. Black/white/2. Loved the story and many of the pokemon designs.
3. X/Y. Don't know why even but loved Gen 6. Also was very accessible for online battles/trading.
4. Ruby/emerald/Sapphire. Gen 3 was very enjoyable. Found it hard to choose between gens 3 and 4 but.. kyogre and groudon helped me decide 3 edged out 4.
5. Diamond Pearl/platinum. Great games. Really enjoyed playing them and very close in personal enjoyment to gen 3.
6. Sun/Moon. Didn't enjoy this gen as much as others. I guess i prefer gyms.
7. Gold/silver. Didn't like any of the pokemon designs, save for scizor. Only pokemon game i didn't do much of the post game for, as Tohjo falls constantly beat out any of the passion i had left for the game. After running out of usable pokemon there for the umpteenth time, i gave up on the game, never to return.

Unapologetic genwunner :P

Top to Bottom
Gen 5
Gen 3
Gen 1
Gen 7
Gen 6
Gen 4
Gen 2

1. Emerald (Nostalgia bias, this is the first real videogame I ever played. But it's also just a rock solid game with a lot of content.)

2. X/Y (Easily the most charming games in the franchise, Kalos is by far the best designed and most interesting of all regions.)

3. HG/SS (Never played the original Gold and Silver so I don't know how these compare to them, but either way, they're great games. Most notably, their side content is the best in the franchise (Pokéathlon, 2nd region...).)

4. Platinum (Don't have much to say about this, it's just a very fun game that does most things right.)

5. B2/W2 (Due to this being a real sequel instead of just a third version, it's the only 'third game' I ever bought. The new parts of Unova often felt more creative than what we got in B&W).

6. B/W (Don't have much to say about these either, just sliiightly less fun than Platinum and B&W2, but still great games.)

7. OR/AS (Maybe it's because I played Emerald too much, but these just didn't feel as exciting. Still solid games. No Battle Frontier was a disappointment though.)

8. S/M (These were definitely a big disappointment. I still enjoyed them since, well, they're still Pokémon games. But they were definitely a huge step down from X&Y. The main things they did wrong were the boring cutscenes and the general focus on (a not so interesting) story, and the bland and uninteresting region.)

9. Yellow (Unplayable outdated garbage. I guess I should try LGFR someday.)

Will rank it by gens, count the remakes within their original games generations.

1. Gen 5. Unova is my jam, with the first one being my favorite Pokemon and its sequel a worthy follower, good stories and characters, all new Pokemon for the first one, stellar OST. 

2. Gen 7. It would be more tied with Black/White if it wasn't for the climax in the Ultra Space, it loses steam after a pretty great buildup for a "final" area and the tutorial is also not good, rest is top notch.

3. Gen 3. Always loved the atmosphere of Hoenn, love both the og and Alpha/Omega, specially the new story bits and more climatic development like in the case with Wally.

4. Gen 1. I really like Kanto still, even if only for the remake or Let's Go, is simple but effective.

5. Gen 2. Would rank this higher if it only was Jotho, going back to Kanto really hurts the overall game, is the most uninteresting thing ever.

6. Gen 6. besides AZ, I couldn't tell you much about this game, really forgettable gym and elite four music, region feels simplistic besides one city and has the worst friends/rivals in the franchise, have very little to care about overall.

7. Gen 4. Bleh, is probably a better game than I would give it credit for, but I hate the combat speed, can't really enjoy my time with it. I really want a remake only for the battle speed.

In all my Years as a gamer I had never played a Pokémon game until Let's go Pikachu lol. I am really enjoying it though even if its not for the purists.

1: Gold/Silver/Crystal
2:Red/Blue/Yellow
3:Everything else

1: Gen 2

2. Gen 1

3. The rest are unplayable.

I'll rank them by category 

Great games:

Mediocre games:

Pokemon Omega Ruby

Pokemon Sun

Bad games:

Best story: gen 5

Best map: gen 2

Best changes: gen 4 (physical and special)

Best water: gen 3 :p

Excluding Gens 1 and 2, because I played them long after they were relevant:

1. Gen 5 (BW, B2W2)
2. Gen 6 (XY, ORAS)
3. Gen 7 (SuMo/USUM, LGPE)
4. Gen 3 (RS/E, FRLG)
5. Gen 4 (DP/Pt, HGSS)

And for individual games: (Not ranked within tiers because I can't decide which ones I like more)

S Tier
Black/White

A Tier
Black 2/White 2
X/Y
Omega Ruby/Alpha Sapphire
Sun/Moon

B Tier
Ruby/Sapphire
Platinum
HeartGold/SoulSilver
Let's Go Pikachu/Eevee

C Tier
FireRed/LeafGreen
Emerald
Diamond/Pearl

D Tier
Ultra Sun/Ultra Moon