What one investment into a pokemon do you regret most?

I regret powering up my first poliwrath, slowbro, and venasaur. I also regret investig all of my stardust on some wild raichu I had in the beginning of the game. I wish someone would have told me to not spend my dust.

Lapras with Frost Breath and Dragon Pulse. At the time it wasn't so bad because Lapras had 2980 CP which was the third highest value and Dragon Pulse, while it isn't Ice Beam, wasn't so terrible to me. Now it really feels wasted given Lapras' current state and how average its charge move is

All of the above with the addition of:
powering up lower-tier mons that I just don't use.
powering up mons past their prestige usefulness.

I also have plenty of transfer boo boos, too.
Some mons that I transferred are now good again because of the moveset rebalances. ETC.

mantra:
yolo!
and
No Regerts!

A Nidoking I powered to over 2000CP before the nerf. He was rare in gyms where I live and back before the nerfs/buffs 2000CP in a gym wasn't too bad.

Now I think he sits at 1800CP. So much wasted dust. Probably closer to 100K.

I also spent a TON of dust on Hypno and my first Elektabuzz. Oops.

I regreat focusing candies on my 2 blisseys that are currently 2900+ cp and still cant get good gym spot in stagnant gyms without getting shaved off.

If i could choose again, i would take all candies back and evolve 5 blisseys and then little by little power up them stronger and use them for gyms that get beated down regularly, where cp doesnt matter.
(only 1-3 players in gym before getting beated down)

This would be best usage for revives + max potions, hyper potions and also max revives, which all of them i have soooooo much.

Also evolving 5 instantly would have show me all move sets i get and i would have better way to choose the best one for further power ups, if i ever get that many candies.

Currently my best moveset from 2 blisseys is P/DG, which is nice, but ZH/DG would have been better ofc.

My first 2 Snorlax...
Now I have 3 higher IV...and 1 higher level that was better idea (now use as one of my main attackers L/BS)

I started playing before IVs were released. Because of this, I powered up a 27% IV Venusaur (in my defense it had solar beam) from 500 CP all the way to 1600 CP.
My Lapras was hit hardest from the stat-rebalance, though it's still a part of my team because of its moveset and 100% IVs.
All my Pokémon benefited/were unaffected from the move-rebalance.
One thing I'm still unsure about is my 47% IV Dragonite. When I caught the Dragonair that pushed me over 100 candy, I couldn't resist the 14 attack stat, and now Dragon Tail/Outrage. Little did I know I would hatch a 100% IV Dratini with 32 candy a week later. I have to constantly reassure myself that the moveset/attack stat make up for the def/hp stat, and she is a fine attacker.

Nothing major. Just minor investments to Nido-royalty, Golduck and Wigglytuff pre-nerf, my first Chansey and Electabuzz back in July because I had candy and didn't know what to do with it. I also powered up my first Exeggutor despite its moves (Zen/seed).

I have more regret over my laughable early buddy choices I chose to help fill my dex. I walked each of: Staryu (42 km), Ponyta (39) Tentacool (15), Exeggcute (21), Vulpix (45), Sandshrew (9), Growlithe (27), Geodude (13, and Voltorb (18). I have multiple of all their evolutions now. So many lost kilometers!

The five first wonders I caught. They did earn a fair amount if coins sitting my lowbie stage though. I didn't take anything last 4500, so it probably wasn't that much of a loss.

I regret three straight attempts (375 candy) to get a nice T-rex and getting IT/FB on all three tries. So I don't have a decent T-Rex. I sent the last two packing simply out of disgust.

My evolves almost always are horrible so when all 3 of my zams over 90 iv all got pc/p I powered them up all the way.

I was able to use them for maybe 2 days before they got nerfed

Hmm, can't say I have much regrets luckily...

I have maxed out a lot of Rhydons since they are common here, but they're mostly sitting in gyms collecting so even better than wasting an attacker to sit in a gym. Gen 4 has Rhyperior so I'd assume it's not a bad thing to boost such a bad defender as well and his stats are still decent/fairly high.

I started the game late (Sep) after kids and wife started (Jul) so I knew all about powerups, mass evolves, IVs, etc...Also, being an old gamer and having played games my whole life and always min/maxing everything, it was not hard to level faster and catch up to most folks even though I consider myself a casual player (mostly play when running about).

I can't believe some people don't even bother with curve balls still (my wife doesn't)...

I'm waiting for them to change Petal Blizzard back to 2 bar so my powered up Venusaur won't suck as much...

I wasted dust powering quite a few Flareons with Ember/Fire Blast.
I also had 3 Charizards with WA/FB, and I powered up the highest CP one.

For me, it was a Pokemon I powered up before I understood base stats.
I powered up a Blastoise because it had WG/HP, thinking it was the same as Vaporeon.

Not a regret so much as a pair of "in hindsight" moments: Lots of investment in my 95% PC/P Alakazam, which at the time was the A+ moveset, too much dust on some pre- FS/O Flareons. I still use them occasionally, but not nearly as much as the investment would have required.

i transfered 100iv machamps because they all had BP... Now i regret because of a possible move change system.

Definitely my first Snorlax, before I knew anything about movesets. Spent all my candy at the time and it has L/E moveset. I never use him now.

I also powered up a B/IP Poliwrath, a C/P Slowbro and a RL/SlB Venusaur before they were nerfed in various ways. Waiting for a few different tweaks to the meta for them to become of use again!

Omastar with 70 ish iv wg hp I thought it could be a good vaporeon but I was so wrong lol I also have another same iv omastar with rockslide and I would have been happy with just that one no wg hp one :/

Also a magmar and rapidash (the rapidash alone is no regret, I just don't need multiple 'low' tier fire types in this meta),
All of these I powered up until lvl 30

a ditto with 80+ iv of low level, I caught like 5 better ones since so that didn't last long