Any game Dragonite is available is the first Pokemon I take to 100. He is probably my 1.5 favorite. The other 1.5 is usually the second, and that is the Fire starter (although I am not completely sold on these Fire-Fighters, made worse by the fact my favorite type pairing has yet to be released, ever).
My first level 100 was a hacked cresselia I trade over GTS for a lvl 1 Growlithe.... They could have at least hacked in Ev's and a few good moves.... BUt my highest level pokemon I have fairly is my level 89, soon to be 90, Empoleon.
My Snorlax on pokemon red but i used the rare candy trick on it, and a lot of others.
I have never got another pokemon to 100 but my Blaziken got close at about L90.