So I've been going over the 'best defender' list on pokeassistant, and their formulas are just retarded. Seems like they put way too much weight on type resistances and not nearly enough weight on hps. I have an upper 20s 97% electrode that, despite ranking so highly on their list, is actually a pretty terrible defender because it only has 90 hp. Here's my take on the best defenders, in no particular order.
Top Tier (Worth powering up)
Snorlax: Ok well this one is in particular order. Nothing defends better than a Snorlax, they're crazy OP. Normal is only weak to fighting, machamp is a pita to farm and nobody is going to power up a primeape.
Vaporeon: Very strong and seems to have about twice as many hp as it should, meaning it's pretty much guaranteed to get off at least one hydro pump.
Gyarados: Hits hard and solid hp. Double weakness to electricity isn't a very big deal because electric pokemon are terrible in Go, at best an electric pokemon can 1 for 1 Gyarados.
Wigglytuff: Solid hp, uncommon weaknesses. Also offensively strong against Dragonite.
Lapras: Multiple vulnerabiltiies but hits hard with very high hp. Also offensively strong against Dragonite.
Exeggutor: Another absurdly overpowered pokemon, hits extremely hard with solid hp. Also one of the best all around attackers in the game. Double weakness to bug isn't a big deal, at best a high level pinsir/scyther can roughly 1 for 1, assuming they take it down before solar beam finishes charging up
Dragonite: Extremely powerful pokemon, double weakness to ice mostly irrelevant because ice pokemon are shit except for Lapras, who is extremely rare. Probably the best all around attacker.
Respectable (Not worth powering up, but still good defenders)
Blastoise: Too much trouble, Vaporeon is better and requires 25 candy to evolve instead of 125
Venusaur/Victreebell/Vileplume: Exeggutor is just better, and only requires 50 candy to evolve instead of 125
Slowbro: Confusion/Psychic Slowbro is a very good trick defender. Guaranteed to stick decent damage on whatever they use to take it down, and all the gen 1 grass pokemon except Exeggutor are weak to psychic type.
Muk: Decent hp and few weaknesses. Unfortunately gets wrecked by Exeggutor.
Clefable: Basically a diet Wigglytuff.