My favourite place would definitely have to be Fullmoon Island. This is where I caught my first and favourite mythical Pokémon Darkrai.
My favourite city is honestly Jubilife City, my favourite music and where you get the Poketch, Poketch apps, PC backgrounds and more.

Smh

My favorite route would be Route 213 and also Valor Lakefront because of the calming music. The houses and the seaside makes the whole area soothing.

My second favorite route would be Route 206 because not only is it a gate to the Cycling Road, it also has a rather beautiful landscape and music (also because I favor the Mystery Zone's music theme)

Favorite town would be Floaroma Town, because it's a really colorful and beautiful town with lots of flowers with variant colors. Not to mention, its night music is really calm and soothing, and the exchanging of Berries for the decorations for the contests make it better.

City would be Snowpoint because of how snowy it is (and because our country doesn't snow at all ;-; ) and because the music is so....... snowy. Like the sleigh bells and the harps and the... sigh it's so calming.

Oh gosh, there's a number of places I like!

The whole Route 209/Solaceon Town/southern Route 210 stretch has kind of a special place in my heart, partly because I've spent a lot of time hatching eggs there, but also because I just like the lay of the land and the overall atmosphere of the countryside.

Hotel Grand Lake/Route 213 is another really nice area. I kinda like that there's a settlement like that right in the middle of a route, and it seems like a very nice beach right next to a lot of lovely greenery!

I love the route to Snowpoint City and also Floaroma town.

shoutout to all the Lake Guardian locations for having super relaxing soundtracks!

As everyone else has been saying, Snowpoint is too good. I love snow. Also it's a great contrast for Sinnoh compared to the hot, sunny, tropical feeling of Hoenn before it.

The Distortion world was so cool. Many things in Platinum wowed me, but entering this place trying to chase Cyrus was just awesome.

Also the field of flowers c:

The flower field and Spear Pillar

I love mountaintop locations in Pokemon and Spear Pillar is my most favorite one of all

The route in between Veilstone City and Lake Valor. Did a good amount of my level grinding there in my last run. The locations where I level grind are usually the ones I have the most affinity for.

I also really like Victory Road in this generation.

I'd say either Snowpoint City or the Old Chateau. Snowpoint City kind of reminded me of my Uncle's place around Christmas and the Old Chateau always interested me since I was a kid.

I like Eterna Forest, the music is pretty.

i personally loved the opening area, it was a nice little small place

The lakes are nice. There aren't many people around, I have a lake guardian nearby, water to swim in... It's easy. Also I like the lake music...

from what i remember i loved the old chateau, Canalave City, Snowpoint City, and Floaroma Town

where do i begin, outside the pokemon league for its gorgeous music. Route 209, Routes leading up to Snowpoint. Snowpoint City itself.

probably my favourite though is hearthome. sometimes if i just woke up at stupid o clock in the morning and cant sleep, i'd throw on diamond and slowly walk around hearthome taking in the gorgeous night time graphics and music, talking to every NPC for the hell of it and taking a random buneary or smth from the PC box into amity square. 10 minutes later i'd be drowsy, shut the game off and sleep till a sensible morning time.

My favorite city would have to be Jubillife city for the simple reason that it showed me how Gen 4 was graphically a gigantic leap from the gameboy advance. What better way to show off than using a large city in the early parts of the game to awe the player.

I've always liked the snowy area of Mt Coronet that you go through when you're going to Spear Pillar. Great music, great background and wild Abomasnow!

My favorite location in Sinnoh, without a doubt, is Sunyshore city. It just reminds me so much of home, and it gives me this really comfy feeling. The upbeat tones of it's theme always fill me with joy and excitement. I suppose I get similar feeling to the one I experienced when I first defeated Volkner, earning my eighth Sinnoh gym badge and getting that much closer to becoming the champion! ♥

honestly i like the first area, its small and quiet and a lake near by it lol

My favourite area is Floaroma town, I love the music and it's just such a cute little place.

I also love Snowpoint city, Sunnyshore city, Hearthome city, Valley Windworks, Lake verity, Twinleaf town, and Eterna forest. :) (Can you tell that Sinnoh is my favourite region?)

I personally love Twinleaf Town, Eterna Forest, and Jubilife City (hearing its track after hearing Twinleaf already wowed me proved to me very early on in my first playthrough that gen IV does in fact have the best music, hahaha) but Canalave City has to be my favorite. The town's atmosphere, Byron, not to mention the theme is probably my favorite music in all of pokemon!

For me it's Floaroma Town. It is a small town with many colorful flowers and it looks so cute and calm.
This is one of my favorite places in Sinnoh!

Ooo, thats a tough one. Probably the snowy routes, they always seemed so peaceful yet upbeat and I absolutely love the music.
I also love Eterna Forest's music theme, along with the Old Chateau and Sendoff Spring.
I just love all the locations in Sinnoh. There was never a dull moment for me.

I love most of the locations in Sinnoh, they're all so memorable: Route 216/217, Eterna Forest, Resort Area, to name a few. I think one of my favourite places though is the Valor Lakefront, the music there is so relaxing and I like visiting the hotel there too. It reminds me of when I lived abroad when it was really sunny and you could just sit by the pool all day at the weekend.

My favourite spot is Newmoon Island. It'sa kinda creepy place with a very good song, and is home to Darkrai. The Member Card event sets the mood just right for the upcoming battle with Darkrai, too.