In the days of Red and Blue, it was either a Marowak or Sandslash (I was too young to care about types, I just used pokemon that looked cool)
I didn't play the second gen 'cos my stupid gold cartridge wouldn't save.
3rd gen was my awesome Sceptile. He was lvl 80 and could completely destroy the Elite 4.
My best pokemon in Fire Red was probably my Blastoise, but my favourite was my Arcanine.
In Diamond, I can't actually remember what my original Elite 4 team was, but after I started trying to fill my pokedex which led me to training pseudo legendaries a lot.
Overall, I actually try to use different pokemon in every game, but would say that my most used pokemon might be a Crobat, I used it in Ruby and i'm currently using it in my Flying monotype challenge, or maybe Scyther, because I always use it in a game whenever I catch it. Yeah, i'm going with Scyther.