Well I'll be glad to edit this post and add to it as I go along, but I just started myself. I did run into a Politoed en route to Alzela town. It's the patch of grass to the left of the water where Fisherman Ralph is located. It was also during mid daytime. 

You can also trade Kyle in Violet city or whatever for an Articuno instead of an Onix

hodow100 : I found a slowking in lake of rage and alakazam in route 42.

Do not forget to include what time of day (morning, noon, or night) you caught the Pokémon, and HOW you caught it (surfing, fishing with what rod, head butting a tree, grassy area...).

I caught a Chikorita during the morning in the first grassy area you find east of new bark town (need surf and waterfall to get to it). Supposedly other starters are there but had no luck so far.
During the DAY time, ran into Cyndaquil in the same grassy areas as Chikorita.
Totodile is at night in same grassy area.

The Alakazam mentioned above can be found in the grassy area there during the morning.

Both Slowking and Delibird are in the grassy areas BELOW Lake of Rage during the DAY.

In the grassy areas below Pallet (have to surf to there) you can find Bulbasaur there in the morning, Squirtle in the Daytime, and both Charmander and Kingdra there at night.

At Mt Silver, east of the pokemon center (or the "Johto" side) You can find Golem there in the morning, Machamp there in the daytime, and Steelix at night.

Clefable can be found east of the National Park's grassy area during the daytime (save a moonstone).

The Kanto starters are found in the patch of grass just below Pallet Town (Route 21). Bulbasaur in the morning, Squirtle in the day and Charmander at Night

the jhoto starters are found in the first grass patch you find on the way to the pokemon league from new bark town and the time of day is probably the same as the kanto starters

ruesen : um ruesen machamp is not located in mt.silver it is located in dark cave. alakazam is the one located in mt.silver.

firewave2981 :
Locations listed here applied to the older version of the game. It has since been updated, leaving most of this information as incorrect. You can find the correct locations on the game page now in the description. I believe our "time of day" for non-cave Pokémon may be correct however.

Porygon2 is now in the grasslands west of Celadon behind the cut tree in the morning.
Scizor can be found in National Park during the morning. No, you cannot catch him for the bug contest.
Gengar is now actually found in Tin Tower at the top at night (at least when I found him).
Celadon Market has moon and sun stones for sale.
Cerulean has the water stones.
Fuschia has leaf stones.

Various other Kanto towns hold the other elemental stones (some repeat).
 
The starters of Johto and Kanto are in the same places and times I believe.
Kingdra should now be at the Whirl Islands.

For fast monster spawning, I recommend getting a Pokémon with sweet scent, use it and save state on the message where it uses sweet scent, then keep reloading until you find a monster you want.Unless you need to level Pokémon, in which case I recommend just running around.

ruesen : thanks for the info I'm sure this is going to help me. I don't use the save states in pokemon games because of the possibility of losing my save file but i don't mind looking for pokemon in the wild and catching them is really easy for me and leveling up i don't have a problem at all.

Found Golem at Night in Mt. Silver.
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Can't find Kingdra while surfing or on ground in Whirl Island ( I've tried in the one with Lugia in it only, so that may be the problem.)
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Chansey is an illusive pain in the rear.
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Machamp, Gengar and Kingdra are the only 'special' Pokemon I haven't got yet. (Haven't tried the first two yet.) 

But yeah, otherwise, this is fun. Trying to literally catch them all for once in my 10+ years of playing Pokemon!