[c-nsp] Best IOS !!!

[Gangasagar Amula]

Dear All,
We are planning to upgrade IOS of all our routers...
Pls let me know to which we shld go for......
A bug free / Reliable / Best performance.....etc..

Equipments used in Network:

 3660 / 3640 Cisco Router : Using VIC cards hence have go for ip plus  feature set.
 Cisco Routers 1601, 25xx series, 1720, 1721, 2610 series --- No Voice....Want to go for simple IP feature.....
Pls suggest.....
Regards,
Sagar.