Manila is warm throughout the year. We're so close to the equator that there's not much temperature variations across the months. In April and May day time temperature is the hottest in the year at mid 30C, so it can be uncomfortably hot for some people. But I like it
December - April is the best time to visit Manila. Day time temperature is around 30C and night time temperature is around 22C. It's also the driest season of the year with sunshine almost every day
Of course, the dry season is also peak tourist season, so consider visiting at the very beginning (late October - mid November) or at the very end of the dry season (early May)
You'll want to avoid June-early November, as this is the rainy season and everything is wet and humid. September and October, in particular, is when the probability of typhoon hits are the highest so you'll want to avoid these months. Typhoons are unpredictable though and it can happen as late as January though that's very rare