stupid question, all pokemon are the same when it comes to exp. you dont find a lv.100 pokemon with 1,000,000exp and another lv.100 with 9,999,940 exp. the fact is that if there was an 'easiest' pokemon, it would most likely be prioritised by people and would be the most popular pokemon, but this clearly isnt the case. you are obviously looking for shortcuts to get good pokemon, but you need to be patient and train properly.
You have to love and train your Pokemon.
Mt. Silver or the Pokemon league
Train them!
Explore!
train, or rare candy
train it
The Pokemon League
there are stations in saffron and goldenrod.
A Magikarp, but I don't know if it is in the Johto region.
If you mean Soul silver, The easiest way is to buy it in the deparpment store
you need to ev train if not use legendarys
trade them to heartgold