There are two Best Attackers by Type guides, but only the legendary-inclusive one mentions future-proofing.
Only the most hardcore players have enough Rare Candy to max out multiples of each legendary that is the best of their type. In less populated areas, even finding a legendary raid (not to mention gathering enough people) may be difficult. Mythical threats are likely to be limited to one per trainer.
How future-proof are the best non-legendary attackers for each type (considering only non-legendary threats)?
Looking at gen 4, gen 5 and expected incoming community days, very few pokemon are safe.
Machamp and Gengar are probably safe until gen 5. Alakazam and Espeon are safe for all generations. Tyranitar, Salamence and Dragonite should stay pretty high up. Metagross with a better moveset would be the best non-legendary steel of all generations, same for Gardevoir as a fairy.
Espeon and alakazam may not be as safe, they're both very fragile (unless mega evolution is added for alakazam) and will have very limited use (ie machamp because not many legendaries or raids are weak to psychic)
Gengar may not be so safe if they release a gengar CD/Breakthrough for October (I dont think its likely, but others have tossed it out there)
Gardevoir cant get a double fairy set and is threatened by togekiss
Metagross will likely get a CD
The rest of the Pseudo-legendaries I agree with
Without looking past Gen 4 because I don't have time to right now, the strongest non-legend psychic attacker in Gen 4 is Gallade. With Gardevoir having the same stats and being an average at best psychic attacker I don't see Gallade making waves against Espeon or Alakazam. What they lack in bulk they make up for in raw attacking power. You can also overcome their glassyness with dodging, though that's tricky with Confusion. The two big psychic attackers in Gen 5 I can think of are Reuniclus and Gothitelle, but without being able to run stats right now I don't know how they'll compare.
Similar story with Gengar. As far as non-legends go the new ghosts in Gen 4 won't even touch Gengar's attacking efficiency. Gen 5's Chandelure will have more bulk and potentially higher attack, giving Gengar a run for its money.
Gen 5's top psychic non-legendaries seem to be Beheeyem and Reuniclus, with 221 and 216 base attack, less than Jynx. Gen 6 has Delphom with 230, still less than Kadabra. And that's it.
As far as top of their type non-legendaries go, Alakazam and Espeon are as safe as can be. Metagross/Gardevoir/Gallade with a psychic CD move is their most threatening competition.
The other response here seems great.
To answer the question not asked: this probably isn't a guide as, while you're correct that getting a full team of the best legendaries is very hard, it's not a big investment to power up non legendaries.
So, aside from perhaps between now and Gen 4 launch (since with the teasers it may be very soon) you'll probably get enough out of any non-legendary you power up if you're doing so for current use. I wouldn't max out a Jolteon now, for instance, since there's nothing you need an electric type for. However maxing a Jolteon to help against Blizzard kyogre when it was out (assuming it could survive a blizzard, hypothetical) wouldn't have been poor decision even though better electrics are coming in Gen 4, because Eevee candy is easily available and the current gain is worth the investment.
Long story short, with all the events and shake ups they're adding, and with how quickly raids can change, ie something like if the raid bosses arent weak to fighting or machamp's taken out of the raid pool, whats good will change fast. Nothing's particularly future proof but the pseudo legendaries (and even then we still have 2 more yet to get events) or things like legacy shadow claw gengar.
You keep using good relative to active raid bosses and that's simply not how it works. Moltres, Entei, and Blast Burn Charizard are all fantastic fire attackers, especially in the sun, but their use outside 2/3 of the regis is pretty limited. When the regis go away they won't be any less good because of it, their use will just give way to more optimized specialists against whoever comes next.
There aren’t many Pokémon that are worth using stardust on. The only ones I can think of are Tyranitar, Machamp, Dragonite, and Blissey. Everything else would be legendary.
Most Pokémon can be caught at level 35, and I would take evolving a level 35 Beldum low IV without using any stardust, then evolving a low CP high IV one and using tons of stardust on a Pokémon with no use at the moment. This same goes for Abra, Eevee (Espeon), Rhyhorn, Charmander, Exeggute, and Magikarp.
I’m not spending anymore dust on any non-Legandary. No reason too other than the ones I mentioned. So you don’t have to worry in any generation about being future proof.
Based on responses to this and other questions about future generations, the future-proofing information in the legendary-inclusive guide and my (very limited) knowledge about future generations gleaned from Bulbapedia:
Bug: Pinsir and Scizor will lose value as Yanmega (Gen 4) and Volcarona (Gen 5) have better stats.
Dark: Tyranitar is very safe.
Dragon: Dragonite and Salamence will face competition from Garchomp (Gen 4), Hydreigon (Gen 5) and Goodra (Gen 6).
Electric: Jolteon will lose value as Electivire (Gen 4) and Magnezone (Gen 4) have better stats.
Fairy: Gardevoir will lose value if Togekiss (Gen 4) gets a suitable fast move.
Fighting: Machamp and Hariyama will face competition from Gallade (Gen 4) and Conkeldurr (Gen 5).
Ghost: Gengar will face competition from Chandelure (Gen 5).
Grass: Exeggutor, Sceptile and FP Venusaur will face competition from Roserade (Gen 4) and especially Tangrowth (Gen 4).
Ground: Rhydon will be inferior to its evolution Rhyperior (Gen 4).
Ice: Jynx will lose value as Mamoswine (Gen 4) and Glaceon (Gen 4) have better stats.
Normal: Snorlax will face competition from Porygon-Z (Gen 4).
Poison: Gengar and Muk will face competition from Roserade (Gen 4) and Nihilego (Gen 7).
Psychic: Alakazam and Espeon are very safe.
Rock: Golem and SD Tyranitar will lose value as Rhyperior (Gen 4) and Rampardos (Gen 4) have better stats.
Steel: Metagross is quite safe.
Please help provide corrections and fill in the gaps (Fire, Flying and Water)!
I'm still of the camp that Gallade won't threaten Machamp or Hariyama without new moves being added as it doesn't learn Counter or Dynamic Punch, both of which are already incredibly strong.
If you're including Ultra Beasts like Nihilego then you may want to see how Pheromosa stacks up against current and future bug and fighting types. Same goes for Kartana (Steel/Grass) and Xurkitree for electric. As for missing types:
Fire: Magmortar (Gen 4), maybe Darmanitan, Chandelure, and Volcarona (Gen 5). Starters with Blast Burn maybe?
Flying: Honchkrow or Togekiss (Gen 4) come to mind by virtue of access to better moves (namely dual flying movesets in general).
Water: ??? It's to judge here knowing how hard Kyogre outranks everything. I guess any of the higher attack starters with Hydro Cannon could claim the top spot.
