Kadabra - Alakazam
Kadabra has absolutely no reason to not immediately evolve into Alakazam as they both end up learning exactly the same moves at the same levels. The only move Kadabra learns through level-up that Alakazam doesn't is Role-play and that's a very gimmick move either way
Machoke - Machamp
This goes for Machoke and Machamp as well. There is absolutely no reason to not evolve Machoke the into Machamp the moment you can, because again, both of them learn exactly the same moves at the same level and Machamp is bulkier, faster and hits harder
Graveler - Golem
Again, there's 0 reason to evolve Graveler at a later state because again, there's not a sinlge move Graveler can learn that Golem can not and in fact, Golem ends up learning more moves than Graveler so evolving it ASAP would even be of your biggest benefit
Haunter - Gengar
Like you might have already guessed by now, Haunter and Gengar learn the exact same moves at the exact same level so you should just evolve Haunter into Gengar as soon as you can because Gengar is simply a much stronger Pokemon than Haunter and it's a true SlimShady.
Other Pokemon that evolve through trade you should immediately evolve
- Boldore -> Gigalith
- Electabuzz -> Electivire (Electabuzz works better as a sole special attacker due to its higher speed and equal special attack stat in comparison to Electivire, but usually you should just evolve it
- Dusclops -> Dusknoir (In-game, Dusknoir is usually more favorable over Dusclops, but competitively, Dusclops is bulkier due to the Eviolite, which make it a better wall, spinblocker and ghost type
- Feebas-> Milotic
- Gurdurr-> Conkeldurr
- Karrablast-> Escavalier
- Magmar -> Magmortar (Only downfall is that Magmortar is slightly slower than Magmar)
- Onix->Steelix (Onix learns Rock Polish at level 19 and Sand Tomb at level 37 while Steelix cannot learn these moves by level-up, however Rock Polish is a TM and do I even need to explain why Sand Tomb doesn't matter)
- Phantump ->Trevenant
- Porygon->Porygon2->Porygon-Z (In-game, you should always evolve Porygon2 into Porygon-Z, but competitively, I guess it's your decision, but Porygon2 is usually more viable than Porygon-Z competitively due to its bulk)
- Pumpkaboo->Gourgeist
- Rhydon->Rhyperior
- Scyther->Scizor (Despite having exactly the same base stat total, you will usually want to evolve your Scyther into Scizor bc it generally has a better typing, better bulk (without an eviolite) and a higher attack stat as well as access to great moves and a powerful priority move, however in-game it doesn't really matter bc of how speedy Scyther is.)
- Seadra->Kingdra
- Shelmet->Accelgor (The only reason why you would potentially evolve Shelmet after level 25 is because of Yawn and 32 if you really want it to have Acid Armor but both aren't exactly necessary)
- Spritzee->Aromatisse
- Swirlix->Slurpuff
Other Pokemon that evolve through trade you shouldn't immediately evolve
- Clamperl->Gorebyss / Huntail (Evolve it at level 50 as it learns Shell Smash at that level)
- Slowpoke-Slowking (Evolve it at at least level 36 in order for it to have Slack Off and at level 41 if you really want it to know Amnesia as well)
