yami's cero didnt seem really powerfull... look at hollow ichigo that went berserk on the outside, while fighting the vaizards there, if you look at that cero, it had tremendous POWER, imagine hollow ichigo if he was consious back than, it would've be even with King Cero of maybe probably ulquiorra or so, and look at hollow ichigo his getsuga tenshou, he was very calm about it and BOOOOM a getsuga tenshou which was 100x bigger than ichigo's getsuga tenshou...
looking at this, yami is just a mouse compared to hollow ichigo...
but... look at those arrancars, they don't have any shinigami powers, they just became more human, they don't have the danger of degradation of their current hollow levels, so they don't need to eath any hollows anymore, they raise their powers just like shinigami do atm.. by experience. arrancars just seal their full powers into a zanpakutou formed weapon... they do not have full shinigami powers yet, like hollow ichigo has, with near shinigami powers they're strong as hell, imagine them reaching real shinigami powers, then you would have a second hollow ichigo..(by power)
but by personality, hollow ichigo would be superior, he just made a stupid mistake getting himself surprised and cornered by ichigo..
aizen doesn't have the personality to keep on going, like ichigo and hollow ichigo have.. hollow ichigo doesn't let anyone play boss over him, but he just made one stupid mistake, costing him being horse again..
now combine his personality with ichigo his personality, than there is small chance of beating ichigo, aizen is just a calm killer who doesnt give a SHIT about anyone, hollow ichigo is a ruthless killer who surely respects others, unlike aizen, but has the personality of a real murderer who won't let anyone take him down, ichigo can't give up because he doesn't want to fail the world..
at my point of view, hollow ichigo and ichigo combined is just the best personality to have in combat, to survive..