National Dex Diploma Explanation

So essentially there are two combos of GBA games and GC games that can complete the National Dex with the minimum amount of games. Both of these combos only vary by their forth game and the the first three are always the same:

Emerald: Obviously a Hoenn game is needed for the Hoenn pokemon. Emerald is the required choice as only using one of Ruby/Sapphire locks you out of one of Groudon/Kyogre and Latios/Latias and cant afford to use both.

XD: Emerald has a set of Hoenn pokemon that it cannot obtain (the Surskit and Meditite line, Roselia, Lunatone and Zangoose) and XD has all of them. One of Ruby/Sapphire cannot be used for this purpose as Zangoose is exclusive to Ruby and Lunatone is exclusive to Sapphire

Its worth noting that XD (and Colosseum) dont have a Pokedex so their goal in a hypothetical National Dex Diploma will simply be to 100% them, which requires purifying all of the Shadow Pokemon

LeafGreen: Obviously a Kanto game is needed for the Kanto pokemon. LG is also notably the only gen 3 game with access to Slowpoke

The last game is required for a whooping three pokes: Qwilfish, Murkrow and Delibird

FireRed Combo:
The biggest issue with this combo is that Emerald would need to complete the game just to trade over its starter, as trading with any non Hoenn game or Colosseum is locked until getting the National Dex. Therefore, Emerald needs to complete the game and reset twice before actually being able to start filling up the dex. Comparatively, one of the Kanto games only need to complete the game (and get to sevii island with 60 caught pokes) and reset once (in this case, for the legendary beasts) and since this takes around the same amount of time as it takes for Emerald to complete the game (2 hours and 12 minutes), Emerald will be around 2 hours behind before it even gets to start filling up its dex. Emerald will be even twice as behind the Kanto game that *doesnt* reset, causing even more issues.

Sidenote: The Johto starters are a non-issue for this combo, as XD can get all of them in one save file. Defeating Mt.Silver rewards XD with one Johto starter and this can be repeated. I didnt know that until recently (I thought you could only get one Johto starter this way) so Im clarifying it here.

Colosseum Combo:
This combo has the exact same issue as the FireRed combo, just with LG instead of Emerald, as LG also cannot trade with any game except FR before getting the National Dex and completing the Sevii Islands sidequest. Do note that with this combo, Emerald only needs to complete the game and reset once (for Latias/Latios), as Emerald can trade with Colosseum anytime. (Emerald would still need to trade over the third starter to Colosseum and then reset but it can do it as soon as it gets its Pokedex (6 minutes into the game) so its a non-issue)

However, this combo is significantly better on the pure basis of only two games needing to complete the Pokedex, which severely lowers the amount of trades necessary