If you already know this, ignore my post and let me know I'm a doofus!
I was planning to catch a ton of Gastlys, basically wanted to get the platinum one... I knew I had a long road ahead of me. I got about 100 or so, then decided it was time I check these ones out. This is where the list in Oak's lab in Stadium is really helpful. I don't think I could have done it in at least triple the time without it. So I've caught about 12-14 at lv 24, 20 at lv 23, and so on till about lv 18-19.
I then put the order in decending according to level, highest at the top. I then realised that the list also puts who is the best overall for that piticular level at the top. I should add that I didn't realise this until I went through about 80 pokemon candying them to level 24. I thought I had found out something really good when I noticed that the first couple of each level went to the top position, or close to, after I candyed it to level 24. Then I realised that the top couple of say level 20 would be the best for that level, so of course it would fair up to the higher levels when it was candied!
Anyway, in conclusion, its probably best to only put the top few or so of each level of Gastlys (I'm just using my example pokemon) and candy them not all of them, saving candies if you are strapped for some reason, adn of course saving time... was there anything I missed? I think so, I'll post again after some replys. I originally thought that there was a thing where certain level pokemon were more likely to have good stats, but that theory went out the window as soon as I realised my goof.