mags was top tier partly because of his infinite, but mostly because his rushdown was insane. imagine how good Abel’s oki is with ambiguous rolls, speed it up, then make it all low risk hit confirms into the infinite. then picture being able to do that at will, no knockdowns needed
cable was top tier because AHVB punished everything for 100% damage and he could hit confirm it full screen, making it so you either approached on the ground (difficult) or from super jump height because he could AHVB normal jumps basically for free. he also could control space like crazy with his grenades and he was a good anti-air assist to boot
Storm is the top tier character I understand the least. iirc she was a zoning runaway monster who built meter easily so she could use one of the best supers in the game whenever she wanted (hail storm). also had a great DHC that made a lot of stuff safe on block and did monster chip damage
Sent is top tier because he did a shitload of damage, could control space like mad. had guard breaks so you could do a 100% combo with mags and then kill the next two characters as they tag in pretty much for free. great DHCs, awesome assist, and he could really lock someone down when he wanted to. he had super armor too iirc so his attacks had ultimate priority
cyclops high tier because he was decent on point, great AA assist, nice DHCs
iron man high tier because of his infinite, proton cannon was an awesome DHC (MvC2 damage scaling eventually reduced any attack to 1 damage, but since proton cannon is something like 45 hits it was always a painful way to end a combo). he had an 8 way air dash too so he had some mixup potential, but his dash was slower than mags so he is easier to block. he could zone with smart bombs and was also a good assist
doom was high tier mostly because of his anti-air (rocks) assist but he could zone too. usually when I see Doom on point though he got massacred. some of his placement is thanks to the strider/doom trap. strider is high tier ONLY because of strider/doom trap
lots of chars are high tier primarily because of their assist. psylocke (combo starter AA) commando (awesome AA that controlled lots of space) and tron bonne (her assist was glitched, if you could get every hit to land it added 45 damage to your combo)
this is mostly theory fighter tho. I don’t play much marvel but I read about it a lot