For actually using? Salamence, was my 2nd Lvl 100 pokemon on sapphire and was great fun, no when using DD/MoxieMence online I love it even more.
For just coolness? Absol, need I say more?
Just talking general favorite. I'm sure most of us have a favorite Pokemon from each gen, in fact I can name my favorite Pokemon I have from each gen except from Gen 4 because I'm not sure on that gen.
Anyway for me:
Gen 1 - Electrode (my favorite Pokemon of all)
Gen 2 - Skarmory
Gen 3 - Slaking
Gen 5 - Chandelure
I like Slaking in Gen 3 because it's really fun to play with those Godly stats and it can pretty much OHKO most things, but for the things that do survive they can take advantage of Slaking when he's loafing around thanks to his ability, which is a little annoying. Slaking proves to be not as useful when it comes to the Elite four because their Pokemon will often survive and hit Slaking hard twice. I also like Slaking's design and find it funny that such a lazy ape could be SO powerful. For those who've played Pokemon Colosseum, the boss you face in that game has a Slaking and that's first when I started to really like it because of his Slaking.
Scyther, Typhlosion, Absol, Lucario, Zoroark. Typhlosion with the overall win though.
Ah, interesting. Quite like Scyther myself, one of my favorite bug Pokemon. I've never been big on starter Pokemon, usually on files I never keep a starter Pokemon for my final team because I just don't like them as much as other Pokemon, but whenever I play Pokemon Crystal, I always pick Cyndaquil and raise it up into a Typhlosion.
And I've just looked the list of gen 4 Pokemon to refresh my memory, and my favorite for that gen would probably be Weavile.