My favorite is Kenmore but sometimes they can be a little more expensive. Stay away from Whirlpool made cheap owned two know wishing I spent the little extra for the Kenmore.
Kenmore is higher end of the 'affordable' scale but they always have top notch quality, on the other end of the scale there are some convection oven/microwave combinations made by Black and Decker & Emerson that are affordable and of good quality.
I know if you go to walmart and want a countertop convection oven they have several microwave/ convection oven combos from emerson and black and decker, i have not seen a stand alone convection oven though.