South West Nigeria is a region of Nigeria. It is mostly a Yoruba-speaking area, although there are different dialects even within the same state.
The weather conditions vary between the two distinct seasons in Nigeria; the rainy season (March - November) and the dry season (November - February). The dry season is also the bringer of the Harmattan dust; cold dry winds from the northern deserts blow into the southern regions around this time.
As with any other region of Nigeria, you would most likely find a reliable coverage of the 2G/3G mobile networks in most states and cities of this region, but reliable coverage of the 4G network might only be limited to some cities or states. Signal strength might diminish further away from cities' centers, and you might occasionally experience a network outage for all or some of the mobile phone service providers in some remote areas in the region. Different networks tend to be more stable and reliable in different locations of the region at different times.
MTN, 9Mobile, Airtel (Zain) and GLO are all available service providers. So if you are planning a visit to any of the southwestern states or cities of Nigeria, you should probably consider having multiple SIM cards, preferably for all the mobile service providers, since you can't tell which will be more stable and reliable at a time in your location during your visit. And if you will risk not having any of them, you might be better off not risking the MTN network, because, from indications, it seem to perform well than the others, though its rates may be high and its offers and services might be stringent, but you are less likely to be concerned of erratic voice and data connection compared to others.
Airtel is another service provider which is fairly satisfactory for all you might need to connect to the world, including rates and network reliability. Of course, Glo (or Globacom) boasts of fast network speed for internet browsing activities, only that you might not be able to enjoy the privilege in some locations. And 9Mobile (formerly Etisalat) usually also performs well in internet browsing speed too but with locational limitations as well, plus data rate is fairly higher.