Not sure why you're still even comparing Thanos with the IG to anyone. The question was Thanos minus the IG.
Consider Apocalypse's resume:
-Beat the Eternal Prime Ikaris, Eternals have easily dispatched the Avengers
-Defeated Eternal Garbha-Hsien
-Defeated Loki
-Defeated Onslaught (for all intents and purposes)
-Defeated Exodus after creating him
-Defeated High Evolutionary
-Created Dracula before later killing him
-Created Mr. Sinister
-Created Archangel's techno-organic wings and granted him superhuman powers
-Defeated Phoenix and Cyclops while they were time travelling
-Defeated Black Bolt + Hulk at the same time and enslaved half of the planet of Inhumans
-Defeated an enraged Hulk
-Manipulated Hulk to be his Horseman and made him more powerful than Juggernaut
-Defeated Black Bolt + Cyclops at the same time, laughing off a direct hit by Cyclops and scream from Black Bolt
Misc:
-In The Twelve, captured and absorbed these 12 mutant's powers at once:
- Bishop
- Cable
- Cyclops
- Iceman
- Living Monolith
- Magneto
- Phoenix
- Polaris
- Professor X
- Colossus
- Storm
- Sunfire
Also brainwashed Wolverine to be Horseman Death and refused his adamantium skeleton.
Had he not been stopped in The Twelve, he would have surpassed the power of a Celestial and slain them all (his goal).
Both Thanos and Apocalypse are at the 100+ strength level according to Marvel wikia.
http://marvel.wikia.com/En_Sabah_Nur_(Earth-616)
http://marvel.wikia.com/Thanos_(Earth-616)
I think in the end it depends on the writer. Some will make them super powerful or stupidly weak. For example, Drax the Destroyer punching out Thanos' heart and killing him (stupid - thought he was immortal?). Or the fact that Thanos can't fly or teleport without this Throne yet can turn people to stone (atomic manipulation). Or Apocalypse being defeated by XFactor or XMen. When you get to characters of these kinds of powers, it starts to get into cheese wiz territory and it's up to the writer to cleverly (or sometimes stupidly) nerf them in order to defeat them.