Both Bellagio and Cosmo are good in different ways -- Bellagio is more classic, Cosmo more modern. Bellagio has an enhanced selection at dinner on the weekend so if you're there over a weekend that is a good time to visit.

I am not sure of this but it feels like Cosmo is better at offering its full selection even for off-peak meals, whereas Bellagio is quite a bit less exciting for weekday lunch than for weekend dinner, so if you're there during the week I'd probably do Cosmo for lunch.

I haven't been to Aria in a while but from what I hear it's not really top-tier any more. If you are looking for another option, Wynn is usually worth a try, though.