Since you are stepping up from a 125cc machine, a logical move would be to upgrade to something in the 160cc space. Here out top picks are the Bajaj Pulsar N160, TVS Apache RTR 160 4V and the Honda Unicorn. The Honda 160s will be a little less sporty than the Bajaj and TVS and will have more upright and neutral riding positions.
If you decide you want something even bigger, then you can consider options like the Bajaj Pulsar N250, TVS Apache RTR 200 4V, Honda CB300F, and Royal Enfield Hunter 350. Of all these machines, the CB300F will feel the quickest, and it is also a rather lightweight machine, which aids its fuel economy and overall manageability.
A test ride will help you figure out which is the right bike for you.