I get where people are coming from saying this thread will make people greedy and **** coz they all be like 'no not worth it YOLO' but im just gonna put my 2 cents worth of shiny value.
the basic stuff to understand is;
chain fished shiny = chain fished shiny
shiny legendary = shiny legendary
But asides from those, you have about 5407956784306843 other Pokemon which dont fit any of these categories, and this is probs where its gonna get confusing.
Each Pokemon is set into its own category - common/uncommon/rare/very rare. I'm sure there are more prominent sites out there which list them, but for me its easy just to use GPX+ because I've been there for such a long time, compelted my Pokedex, so I can easily search for the classification of any Pokemon (though it hasn't updated w/ 6th Gen yet, but there are only about 60 off Pokemon, so shouldn't be too hard to classify based on previous classifications)
I was going to upload dex pics of each category so you can see which Pokemon is in which category, but I can't be bothered (if people want me to I will)
That said though, there are certain things which need to be taken into account - such as shiny color, and stage +/-1 (i really didnt know what else to call it lol)
shiny color:
its so obvious that the shiny colouration of a Pokemon makes it more wanted. i know people really love the budew/roggenrolla/axew/kirlia shiny evolution lines, but roggen/budew are classed as common, whilst axew/kirlia are classed as uncommon. Because of this, i'd say they should be upped a tier due to being wanted more due to their shiny color.
and then we have pokemon without distinctive color change - ie; meowth and togepi (they're really the only two I can think of). Meowth is classed as uncommon, and Togepi classed as rare, but as their shiny colorations aren't that different to their regular colors, their value goes down (so meowth is now common, and togepi is uncommon)
stage +/-1
this is more of a common sense thing that I use which basically corrects pokemon to their proper rarities because i dont think they belong in the rarities they're under.
For this i'll mainly use the rare tier as an example because its the shortest so is much easier for me.
Now given that list, there are so many thing I would change about it.
First of all, the only Pokemon that would definately be in the 'rare' list for me would be the starters/pseudo legendaries/other (because they're hard to get, or can only be obtained near end/post game) I feel as though I should list them all and then explain so maybe I will do that because that'll be easier (but i'll remove evos so it shortens the list) - I wont put in a spoiler either so it's easier, but if it stretches the page too much let me know and I'll put it in.
Bulbasaur
Charmander
Squirtle
Vulpix - uncommon
Kangaskhan
Tauros - uncommon
Lapras
Eevee
Porygon
Aerodactyl
Dratini
Chikorita
Cyndaquil
Totodile
Togepi - uncommon (as previously stated)
Shuckle - uncommon
Heracross - uncommon
Skarmory - uncommon
Stantler - uncommon
Miltank - uncommon
Larvitar
Treecko
Torchic
Mudkip
Slakoth - uncommon
Nosepass - uncommon
Torkoal
Zangoose - uncommon
Seviper - uncommon
Lunatone
Solrock
Feebas - uncommon
Castform
Kecleon
Tropius
Absol
Wynaut
Relicanth - chain fish
Bagon
Beldum
Turtwig
Chimchar
Piplup
Happiny - (shiny coloration is pre much the same) - uncommon
Spiritomb
Gible - NB: even though shiny coloration is pre much same, its a pseduo legndary, and mage garchomp is purple so it can stay as rare
Munchlax
Riolu
Rotom
Phione
Snivy
Tepig
Oshawott
Munna - uncommon
Sigilyph - uncommon
Zorua
Ducklett - uncommon
Emolga
Alomomola - chain fish
Tynamo
Cryogonal - uncommon
Golett
Rufflet
Vullaby
Deino
Larvesta
that's just what I would do because that's how I'd rate them
and that's how valuable they are to me
but all in all, it really comes down to the traders. If they're willing to negotiate, as long as they'er happy with what they're both getting, it shouldn't matter what they trade.
If said person really wants a Pokemon in the common category, and they're willing to trade a rare tiered pokemon - then who really cares.
As long as they're both happy with what they're receiving then it shouldn't matter IMO. Hence why I often decline shiny offers when I get trade requests, because I'm simple not looking for that shiny.
also once Pokemon bank goes live, I'm going to try to only trade from Pokemon which haven't been transferred, just so rarity isn't affected as some pokemon are easier to get in previous games, but harder to get in X/Y.
that being said, stats and all that **** are also taken into account. someone might be looking for a shiny Pokemon to use competitively, and this Pokemon might be classed as common, but be a good Pokemon to use in completive battling, thus its rarity increases, despite being classified as common. Plus, an uber Pokemon like Gible which is rare might might be traded, but it has **** stats, so who'd want to use that since Garchomp is good at battling? It further decreased value.