I find it interesting and sort of amusing that there are like 100s of scents that I never experienced because either they were released before I was born or I wasnt in the hobby yet, so I ask, what are your favourite ones! Mine is probably 
Miss Dior Chérie (2005) Eau de Parfum that a family member gifted me a small shoping mall sample from 2005
Piazza di Spagna. I don't know why it got discontinued and struggle to describe what's so special about it, but it's my favorite perfume ever. I've been holding onto my precious little bottle for more than 15 years now, lol.
It's fresh, it's spicy, it's... I'd say herbal? I've owned it for so long that I genuinely struggle to describe it. It just smells like itself, and I've never come across anything else like it. Best thing ever, 10/10.
The only two discontinued fragrances I truly miss would be:
- 
A*Men Ultra Zest : The only Mugler I actually like with that burst of Blood Orange in that A*Men DNA you could wear all year round. I have not found a dupe (or clone) that comes near it. 
Ultrared Man does somewhat tap out of the same vein and fill the void to some extent, but it is not the same.
- 
La Nuit de L'Homme Bleu Électrique came with a flash and went out like a thief in the night. I thought it was well received but was discontinued pretty fast. I'm glad I stocked up on it, but still: why this was taken out of production still vexes me.
There are far too many to name, but here are a few highlights:
Marc Jacobs Eau de Parfum I only have one tiny bottle of the perfume that I never use.
Rose 4 Reines Eau de Toilette I have half a rollerball full. Never got a chance to try 
4 Reines Eau de Parfum
Stella Eau de Parfum Also never had a chance to try.
1842 Rosa Alba I sold off my mini and I regretted it.
More discussion about the same subject can be found in this thread. ☺️
I wore Weleda Kölnisch Wasser (renamed Everon towards the end) for decades. Was very disappointed when it suddenly vanished in 2001. I called the Weleda home office and was told 'this was a vanity product and they were concentrating on the therapeutic line.'
In recent years they began offering a couple of women's colognes but they've never made the original unisex scent again.
I have used their natural Sage Deodorant since the 1970s.
One of the things I love about this hobby is the hunt for discounted/unicorn fragrances. Discontinued doesn’t mean unavailable as long as you don’t get your pants pulled down over the price! I have far too many discontinued loves to list but I would say it’s what makes this great. I would go further to say that hunting out 1 st formulations etc. of fragrances still in production is also amazing, you get a little slice of olfactive history and smell better than your peers, wearing the same modern watery shadow of a fragrance's intended experience.
One of the things I love about this hobby is the hunt for discounted/unicorn fragrances. Discontinued doesn’t mean unavailable as long as you don’t get your pants pulled down over the price! I have far too many discontinued loves to list but I would say it’s what makes this great. I would go further to say that hunting out 1 st formulations etc. of fragrances still in production is also amazing, you get a little slice of olfactive history and smell better than your peers, wearing the same modern watery shadow of a fragrance's intended experience.
Hahah, great that you're seeing it in a positive way 😅 I must say for me it's the opposite. One of my least favourite things in this hobby is that I constantly need to worry if my favourites will run out and if I have to pay crazy prices to get backup bottles if they have been discontinued, that is soooo super stressful for me personally. Probably it depends also on how much spare money you have and where you live - if I was rich and if I lived in a country where even discontinued things are easily available with decent shipping & customs costs, then I wouldn't worry that much at all. 😄
Ambre Gris - one of my first loves and still a favorite. At the moment bottles still show up online at a higher price, of course, but I'll probably just appreciate this final bottle I'm on for as long as it lasts and then say farewell.
Midnight Fleur - I think most Nest fragrances are pleasant enough, but Midnight Fleur was something else. I still can't believe they discontinued this one, but I guess it just wasn't as popular among the masses.
I have extremely fond memories of Calvin by Calvin Klein in the 80’s. But I was much younger then. Perhaps it’s not as great as I remember. Haven’t smelled it since the 80’s.
Would love a bottle of the original poisoned plum in 
Mon Parfum Chéri par Camille Eau de Toilette. And for a cold winter day, the playful, crispy pomegranates of 
Quel Amour!.
Replica Matcha Meditation by Maison Margiela. Creamy and delicious matcha latte with just that hint of white chocolate...
I used to love this cologne spray that holister had back around 2012. It was called "Newport Beach" I was in middle school and the teachers and students gave me compliments on it. I really wish I could find a bottle of it again. It was beautiful and I wish there was something similar realesed today
I think a good argument can be made for saying that some of the great fragrances of the past that have been reformulated should be considered as discontinued. Just recently I purchased the current version of ...
and
I enjoyed both, but I can tell that they are different to what I remember them being in the 1980s and 1990s when I wore them regularly. However, we have what we have and there's no going back, so there isn't much point in discussing it.
One discontinued fragrance I'd particularly love to see return is ...
I first purchased this when it came out in the late '90s and it was a favourite of mine for several years. Yes, OK, it has a slight rubber tyre note in the opening, but as I am a fully paid-up petrol head I do not mind that at all. Thankfully, I still have a bottle from about 17 years ago that's about 30% full and I use it sparingly.
Some others from the 1980s that I'd like to experience again are ...
Nino Cerruti pour Homme Eau de Toilette
^ I first bought this in around 1984 and I remember not liking it much. As I suspect my taste has changed a bit over the years, I wonder if I'd prefer it now.
In the latter part of the 1980s I loved this ^ aftershave.
Chrome Extrême still have a bottle though
The reason I will forever despise LVMH with the fury of a thousand red hot suns.......Joy by Jean Patou.
Not only did they kill off a classic, legendary perfume, but they stuck the name Joy on a third-rate new perfume by Dior.
Closest thing to sacrilege in Perfume World IMNSHO. I have a small decant of the EdT, and hope to scrape up the coins for a Christmas bottle from the remaining stock out there.
Dzing! the circus has left the town
Yes!
Also the trio 
Safran Troublant
Poivre Piquant and 
Piment Brûlant .
Criminal.
And the only good Tom Ford is the one they discontinued, obviously: 
Violet Blonde
Im also bitter about 
Gucci Eau de Parfum, 
Daphne , 
No. 23 , 
Eau de Polder, 
Rubj Extrait de Parfum and a bunch of others I didn't have the money for at the time.
Escada (1990) 
Dewberry 
L'Eau de Monteil
Zibeline (1927) 
Amour Nocturne
Charlatan
Shocking 
Womanity 
Fève Délicieuse
Alien Essence Absolue
Bois des Îles 
Santal Nabataea
Tea for Two
Flora & Fauna
Sous le Vent 
Scandal 
La Nuit 
Le Temps d'Une Fête
Mr. Burberry Indigo
Dark Rebel
Dark Rebel Rider
Nirvana Rose
Antaeus 
Pour Monsieur 
Bvlgari Black
Xeryus 
Elements Aqua 