Favorite:
I would say Espeon, little weaker than Alakazam and learns less moves, but my favorite otherwise.
Least favorite:
I cannot speak for the weak pokemon I have not used, but the one weak pokemon I have used and hated is Wormadam (trash cloak), I hated it so much that I kicked it out of my current party for a Rhyperior.
Favorite type:
Psychic and Dragon by a close margin. Psychic because the first evolved Pokemon I used was psychic. Dragon is also my favorite because it is quite a unique type (a little overrated, I might add) and Pokemon under this type are generally very powerful.
Least favorite:
Bug is by far my least favorite, with a few exceptions (Scizor to name one). However, I am often hesitant to use the exception bug types, because, even they, are easy to beat.