Loki's strength isn't straightforward. His power lies more in his intelligence and ability to manipulate situations. He can outwit many opponents, but in raw physical strength, he might not be the strongest.
Rimuru's strength in the light novel is quite significant. He acquires unique skills, builds a powerful army, and overcomes many formidable enemies. His power grows exponentially as the story progresses.
Issei's strength in the light novel is significant. His power increases through training, experiences, and unique opportunities, making him a formidable character.