The biggest arguments for Fedor are:
1. He was generally outweighed by his opponents. Say what you want against HMC, but at the size differential is something no one in the post-sanctioned UFC has faced.
2. He often fought with a ferociousness and reckless abandon that gave him fearful aura of "I'm going to destroy you.", compared to A. Silva's "you simply can't hit me unless I let you" or GSP's, "Anytime you try something I'm going to take you down" or JJ's, "I can hit you at a distance you can't touch me with, but if you get close to me I'm going to elbow, knee and throw you.".
3. He fought at HW where long-winning careers are almost unheard of. I think the UFC HW champ's consecutive title defenses is still 2!
4. While he had an incredible work ethic and his pudgy body shape wasn't as detrimental as it looks, he had a sort of amateur approach to fighting and training that was commendable. Maybe I'm just fooled by the hype, but it seemed like he reclused himself to the middle of nowhere in Russia, did cardio on playgrounds, ate traditional Russian foods cooked by his babushka, trained with his brother and random Russians sambo practitioners, and every few months would come out and destroy the best the world has to offer. He didn't seem to have a Greg Jackson to run a 'camp', he didn't have a Cus D'Amato to mentor him and teach him the sport, he didn't have a Mike Dolce to figure out an optimal diet and I never even saw him with competitive training partners that had better pure boxing, kickboxing, muay thai, JJ or wrestling than he had.
5. His gameplan was often not to play to his strengths and force the opponent to fight his gameplan, but often fought his opponent's style and fought to their strengths and beat them at their own game. A. Silva was a boring point fighter if you didn't let him counter-attack. GSP would take you down and smother you if you didn't give him opponents to GNP. JJ somewhat follows Fedor's footsteps in this in that he outstruck Glover, Gustaffson, Machida etc and outwreslted Cormier. When Fedor fought Big Nog, Fedor's plan was to jump into Nog's guard and avoid getting submitted (same as his strategy against Werdum!). When Fedor fought Crocop, he kickboxed him.
IMO The biggest argument against Fedor is that prime Cain beats a prime Fedor. I think Cain would just be able to smother and outwork Fedor, avoid submissions and outwrestle him to victory. I can't say the same of a prime GSP or Silva.