Grass type was definitely the best, seems the 1st, 2nd, 3rd, 8th, EliteFour #1, #2 was all beyond it’s control. The 1st, 2nd, 8th gym (rock & water & ground) was taken 2x effective damage from grass type, and electric was not so effective to grass type. Same as Elite 4, the 1st and 2nd one has some Pokemon that was taken 2x effective damage like Loreliei’s Dewgong, Cloyster, Slowbro, and lapars and Bruno’s Onix.
One of the best in game grass type was Venasuar, our grass type starter which we could obtain early game (or just say the starter).
Why electric? Electric has only 1 weakness and has advantage on Falkner’s all Pokemon, Bugsy’s Scyther, Chuck’s Poliwrath, Pryce’s Seel and Dewgong, Brock’s Omastar and Kabutops, Misty’s whole gym except Quagsire, Janine’s Crobat, Blue’s Pigeot and Gyrados, Will’s Xatu(es) and Slowbro, Karen’s Murrow, and the champion’s Gyrados, Charidzard, Aerodactyl and a bit on Dragonite. Almost every single gym has disadvantage to Electric! OMG!
Mareep will be the choice for it, which could obtained early game and has a pretty good stat after evolve.
Gen 3 (Ruby and Sapphire):
Fighting has the most type advantage early game and elite four, which has advantage at Roxanne’s whole gym, Watton’s Magnemite and Magneton, The Toughest gym ever in gen 3 —Norman’s gym with slakings , Wallace’s Sealeo, Sidney’s entire gym, Glacia’s entire gym, Steven’s Cradily and Aggron.
A good fighting type Pokemon which could obtain early game was Breloom, who’ve got three amazing ability and movepool was also good. It could also immune to Powder moves and leech seed which was AWFULLY AWSOME, bonus grass type typing helps you also got advantage at Wallace’s whole gym and Steven’s Claydol.
Gen 4 (DP):
Fire type has slightly more advantage at gyms then others, it could get rid of Gardenia’s gym, Maylene’s Lucario, Sweep Toughly (Cuz of steelix) at Byron’s gym, Candice’s gym, Aaron’s entire gym (Except Draipion), Flint’s Steelix, Lucian’s Bronzong and Cythia’s Roserade and Lucario.
A good fire type Pokemon in DP will be Rapidash if you don’t choose Infernape (which most people don’t ), which could at least obtain before Eilite 4 and has a good stat.
Gen 5 (BW):
Rock type was a good one for gym sweeping in gen 5, which has advantage at Cilan, Chili, and Cress’ Lillipup seems it’s a resistance, it was even more better if you choose snivy, it’s also has advantage at Lenora’s gym seems resistance, Burgh’s entire gym, Elesa’s Emolga, Skyla’s whole gym, Brycen’s whole gym, Shauntal’s Chandelure, Froslass and Drifblim, Caitlin’s Sigilyph, Alder’s Accelgor, Vanilluxe and Volcarona, N’s Archeops, Vanilluxe and Reshiam (If you play White).
A good rock type will be Archeop, who has super good stat if HP was High, Deafeatsist won’t disrupt it from the sweeping.
I will write down the other generation soon if I has enough time, and hope it helps!