Here's the ones I found useful so far and later. (Looks pretty good to me...)
Abra-Mr. Mime (Route 2)
Nidoran F-Nidoran M (Route 5) [This trade might work the other way in FR, but I was making it easy for myself by just catching another Nido F and trading it. Plus exp. bonus...]
Spearow-Farfetch'd (Vermillion City)
Poliwhirl-Jynx (Cerulean City, only way to get her that I know of...)
Slowbro-Lickitung (Route 18, again, only way you can get it) [Make sure to get 2 Slowpokes in case you want Slowking.]
And I don't know of any others that strike me. Especially if there are different ones that I haven't figured in FRLG.
Hope this helps!
It's great to complete a Pokedex! So I'll need some help...
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