Volcarona, Braviary, Beheeyem, Eelectross, Golurk, Escavalier are all pokemon that I've run in the past and believe to be good. Honestly this is a pretty subjective question :P It reallay depends on the trainer and how they like to play the game.
I have Pokemon Black, and as far as i know you cannot catch event legendarys without an event (eg. Genocect, Zorua). You cannot catch some Pokemon on the game, but Pokemon White can, and White can't catch some Pokemon that you can catch on Pokemon White. Lol!
cheat ar or some sort of an event
some show up in white forest
I know there are others, but here are some examples: Pokemon Black-Gothita, Vullaby, Cottonee, and their families Pokemon White-Solosis, Rufflet, Petilil, and their families
Some say that you can find it in flocessy ranch or in a route with about 3 ledges in a row in pokemon black 2 without any cheats
at stark mountain
There are no codes
Milotic, Empoleon or Vaporeon.
not at the begining. if you finished the game you will be able to catch old Pokemon too.
pigeot charizard zapdos moltres articuno fearow
Yes you can, If you have completed the game you will be assigned the national pokedex and once you have that you will be able to catch every Pokemon ever made in the wild, except legendary's but some might be harder to find than others.
Poochyena is a great Pokemon to catch. At first, he doesn't have too many attacks, but after leveling up some he is really good, especially after evolving into Mightyena. Poochyena can also learn rock smash and strength, which are TMs you will need later on in the game to reach some areas. Ralts is also a good Pokemon to catch, if you want a psychic. Taillow is good, especially because you will need it for fly, and it can learn steel attacks.