Keeping Previous Ascensions?

I plan to keep Jack's 1st ascension mode because I am not a degenerate...

There are plenty that I change the ascension stage for.
Jack keeps her cape
Mary/anne rider aren't in the bunny girl outfits
I prefer kiyohime with white hair, so I don't use her final saint graph.
HATE liz's frilly pink outfit... I use 2nd stage
I prefer Mordred in armor.
Nitocris is scandalous enough in early ascensions, don't need to go to the 3rd
DaVinci's hat is weird, she stays 1st with me.

And I'm probably forgetting several...

Jack, TamaCat and TamaCaster on 1st ascension
Medusa, Medea and Robin on 2nd ascension

Iskandar's pre-ascension art and his second stage outfit is much better than the 3rd stage/final stuff. It actually looks like he's ready for battle in second stage.

There are a few others that I prefer in first or second stage as well like Lancelot Berzerker.

Why not? I always keep Waver at 1-2 Ascensions, ever use 3rd and final. I am glad many my friends also consider 1-2 middle-aged Waver are better than young lord.

Some of them in 1-2 ascensions are still great. Like I love to see Archuria 2nd's smile. Also I always keep my beloved Tamamo in 2-3 Ascensions since I wont let her cry in the final.

I keep Atalanta on 2nd ascension (sprite, art, icon, command card) because I love her in /Apocrypha and 2nd ascension outfit just looks the best
I also (like guy above me) don't like Liz's pink dress on 3rd ascension but I love 4 ascension art so I changed only sprite and command card, icon and art stil are 4 ascension.

About servants I don't own but... [all servants listed here in my opinion were looking better at some point rather than on 4 ascension]
Tamamo Caster - 3rd ascenion all elements, previous look a little poor (but I understand nostalgia to /Extra series), 4 is pure hentai (or at least ecchi)
Merlin - 4 ascension splash is bad, anything beside this is nice
Gilgamesh - I hate his ABS, definietely everything changed to previous ascensions
Jeanne d'Arc - 1 ascension ftw! /Apocrypha lover here ~
Mordred - I'd change icon and art to 3rd ascension, 4 looks.. weird

Nothing more comes to my mind...

I keep most of the FSN and F/Z servants at their first ascension forms since that's how they appeared in their original works. The rest don't bother me much.

And Illya stays in her first form because ^_^

I keep Martha Ruler on third stage art since I like it more than final.
There's nothing good on Lancer Liz apart from default.
Emiya's on default because something about it seems better than his third.
Nero's lion is ridiculous (who even thought that was good?)
Young Waver for height difference with all my "tall" servants.
Ibaraki's on her first art since I don't like all the rope n' stuff

I only do it for Mordred. She's not a gag like summer characters so why TF would she ditch the protection her armor gives for some casual attire when in battle? Plus, her armor is awesome! So, I just leave her on sprite 2 so I can see her beautiful face while brandished in fine protection. However, I leave her saints graph as her F.A as it's far more fitting that the attire she'd wear around Chaldea be something casual.

I keep karnas second Ascension. I dislike his lance. It steals to much screen. I also don't like jalter's or normal Jeanne final ascension's either. I usually keep the first or second.

Totally get that. Raiko will always stay with the 1st Ascension's look, hate all the stuff she gets. Support you on the Eli staying with the looks of the 1st and artoria archer of the 2nd.
Also, keeping artoria lancer alter with armor, no helmet. Don't want a half naked woman on a horse.

I keep summer artoria on ascension 2 and summer Martha on ascension 1.

Marie Antoinette Rider 3rd ascension, 1st and 2nd ascension is a Thug Life mode 😎
OG Nero 1st/2nd ascension Fate/Extra Last Encore.
OG Arthuria 1st/2nd ascension, Fate/Zero.
OG Chu 2nd ascension, he is looking more fierce.
Hans 1st ascension, using hands to cast skill and NP is cool.
Gilgamesh 2nd/3rd ascension, Fate/Zero and muscular Zasshu LOL 😆
Li Shuwen 3rd ascension, Triad mode.
Tristan 1st ascension, i'd like him with his shirt that he wear in Camelot.
Robin Hood 1st ascension, hooded Robin of Locksley A.K.A Faceless King.
Leonidas 1st ascension, remembering the epic battle while he fought immortal, Battle of Thermopylae and of course : THIS IS SPARTA ! Leonidas shielder when ☹️
Mozart 2nd ascension, i'd like non-masked Amadeus looks like he is playing Fur Elise for my D'eon.
D'eon 1st ascension, best paired with Mozart because they have good taste of music ( his/her NP is Fur Elise ).
Arthuria Lily 3rd ascension, nothing much to say i just like her on that Costume.
Lu Bu 3rd ascension, it's LU BU RUN ! When you have PTSD while fighting him on Dynasty Warrior. Also Lu Bu 5 star Rider and 5 star Lancer When 🤔
Carmilla 3rd ascension, i'd like non-masked servant, hat is okay but no mask.
William 3rd ascension, he is a gentleman in that suit, cool.
Siegfried 2nd ascension, Fate/Apocrypha. Sigdrifa when 🤨, well there's Sigurd and Brynhildr, so there's Sigdrifa as well.

Stage 1 Caster Marie (gotta keep the crab)
Stage 2 Medb, d'Eon, Marie, Cu, Kid Gil

I put ibaraki on 2nd form because I didn't like that headgear she got on her 3rd form...

I will never use scathatch lancer final SG as it's so not scathatch like for me...

I also prefer to use mordred rider 1st form.. because she is the best that way for me...

I prefer cu chulainn lancer on his 1st form... maybe there's more vut I just forget them...

I keep everyone on their final ascension art just because I got them there, so it feels best to have them with their fancy art in my teams and such. However I'm not strict about their actual in-game sprites:

OG Artoria - 1st Ascension. I love the invisible sword aesthetic.
Archer Artoria - 2nd Ascension. All of her outfits are completely different and the 'sporty' one looks really good on top of not being over-the-top like her third one.
Lancer Artoria - 1st Ascension, because Camelot is just so good so I like having her look like the 'Lion King' with her helmet.
Cleopatra - 2nd Ascension, because obviously you gotta keep the MJ look.
Jack - 1st Ascension for hopefully obvious reasons.
Cu Alter - 1st Ascension, because I don't really like the upgraded sprites when it feels like they're adding stuff for the sake of adding stuff.
Jalter - 2nd Ascension. Short hair look is the best and gets rid of her awkward looking cape thing.
Dantes - 1st Ascension, I like the simplicity.
Siegfried - 2nd Ascension. The dragon stuff is just...too much.
Artoria Lily - 1st Ascension. Why get _rid_ of the armor?
Emiya - 1st Ascension. The darker coat seems like it meshes better with the rest of the game.
Nursery Rhyme - 2nd Ascension for the relative simplicity, but I also keep her cards in 1st Ascension because I like the look of the books.
Kid Gil - 2nd Ascension. The red cape is way too much.

Waver third cuz im no shotacon
JAlter first cuz zettai ryoki and no cape