What determines the best Pokemon team for pokemon fire red, and leaf green? In my opinion it is whatever team you have the most fun with. Although I only play against the single player campaign, and rarely fight anyone in pokemon. I usually have fun even though I may lose because I choose a team that I like using the most?
Now others may have a different opinion then me, and the best pokemon team might be the strongest pokemon team out there other wise they do not like the pokemon team. That can also be how people choose the best pokemon team for them. However the real best pokemon team anyone can have is the pokemon team that you personally have fun using, other wise what are you playing the game for if you are not having fun.
Some people may disagree with this being the best team for them, but this is personally my favorite Pokemon team. The following pokemon are my person favorite Pokemon team Gengar, Charazard, Gyrados, Lapras, Mr Mime, and Mewtwo. Yes some of these pokemon are actually really good pokemon, but that is not the reason why I chose them let me explain.
The main reason why I chose these pokemon is because of personal experiences with either watching lets players play pokemon or my personal experience with playing the games. For example when I watched a Pokemon episode with a giant Gengar from then on I liked Gengar, then the 4th gym leader in the pokemon gold, and silver series made it more fun to play with one.
Basically that is how I chose all of my pokemon through past experience, and child hood memories. Like before this is the way I chose the best Pokemon team for me, and others may have there own method to doing so.