What does "legitimate" mean in this case? Does it mean 'refraining from glitches, Rare Candies, etc.', as in leveling up purely through training/battling?
I have no idea which one it is exactly, seeing as it must've been about 10 years ago that I got my first one to 100. One thing I'm sure of though, it's definitely one of the starters in Blue, since back then, they were my top priority on the team. Since they'd been with me the longest, they were usually the highest levelled, so they also usually made the first in my team in important battles, and thus came a long way ahead. I'm pretty sure I really preferred the Charmander line back then, so my first to level 100 would most likely have been Charizard.
Yeah; my starter Pokemon are typically the Pokemon with me the longest and often end up being the highest-leveled Pokemon I have with me at any given time (and they're usually able to combat most threats effectively, so they're usually ordered first in line in my Pokemon party). As a result, my first Pokemon that attained Level 100 in battle is likely my Blaziken of my first
Pokemon Sapphire game. Either Kyogre or Rayquaza or one of the other starters was probably next, especially since they were caught at high levels.
I've never gotten a Pokemon to level 100. I rarely get them much past level 50, and even that's the exception rather than the rule. There's just always something else that I want to breed or catch or train or evolve or what-have-you, and that's my priority. If they're particularly good or I just like them, I put in the effort to breed or catch exactly what I want, then level and train them for as long as it takes to get what I want. Generally though, once they're fully evolved and fully EV trained, I only use them if I really need them on a team - otherwise I'm focusing on the new stuff that I want to breed and catch and evolve and train.
I have boxes and boxes full of level 30-something fully evolved and EV trained Pokemon with carefully designed movesets.
Not sure if you're the competitive sort (as in, Link/Wireless/Online Battles), so it seems strange that you'd go through all that work designing movesets and applying EV spreads but still not eventually train the Pokemon to Level 100. I've heard about an "auto-level" feature in a few recent Pokemon games, though, so I woudl think that the feature discourages a lot of people from training Pokemon to Level 100 when the game can technically do it for them. :/
I usually use Rare Candies (if I happen to have them) right after a Level-Up, unless I'm saving them for a later purpose. My legendaries typically get trained to Level 100, especially due to being at high levels already (which really helps when I'm engaged in a battle with one of those annoying Trainers that I forgot I snuck past pre-Elite 4+Champion Defeat).
I haven't trained a Pokemon from Level 1 to Level 100 yet, lowest beginning level so far has been at least Level 5 (due to Generation 3 Eggs starting at Level 5); might have even been Level 2 or 3, possibly being one of those Hoenn Route 101 Pokemon...
Oh, and in b4 someone mentions "catching one of those Level 100 Magikarp in
Platinum Version". :P