For exploring Buenos Aires with the hop-on hop-off bus, March, April, May, September, October and November are usually the most pleasant months. During that period, it is often more comfortable to step outside, walk around neighbourhoods for longer and avoid too much extreme heat between stops. That makes it easier to really experience the city.
In the hottest months, the ride can still be manageable, but getting off and walking around feels more intense more quickly. In cooler or wetter periods, the city may feel calmer, although you may want to take clothing, wind or rain into account a bit more. If you want to explore comfortably, it is best to choose a month when being outside still feels relaxed.