The definition of "exp farm" was never very good, so I'll just list the top 4 non-legendary non-mega Pokemon with the most base experience in each generation. Please tell me if this is not what you want.
Gen 1: Chansey (255), Lapras (219), Dragonite (218), Gyarados (214)
Gen 2: Chansey, Blissey (255), Lapras (219), Dragonite and Tyranitar (218)
Gen 3: Chansey, Blissey (255), Lapras (219), Dragonite, Tyranitar, Salamence (218)
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Gen 4: Chansey, Blissey, Happiny (255), Togekiss (220)
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Gen 5: Blissey (608), Chansey (395), Audino (390), Dragonite, Tyranitar, Salamence, Metagross, Garchomp, Hydreigon (270)
Gen 6: Blissey (608), Chansey (395), Audino (390), Dragonite, Tyranitar, Salamence, Metagross, Garchomp, Hydreigon, Goodra (270)
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Gen 7: Blissey (608), Chansey (395), Audino (390), Dragonite, Tyranitar, Salamence, Metagross, Garchomp, Hydreigon, Goodra, Kommo-O (270)
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Gen 8: Blissey (635), Chansey (395), Audino (390), Dragonite, Tyranitar, Salamence, Metagross, Garchomp, Hydreigon, Goodra, Kommo-O (300)
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