The subtropical climate in Guangzhou allows travellers to go to the city anytime of the year, so when to visit will depend on what you prefer to do. If you want to avoid the exodus of humanity then you should stay away from the Chinese holidays, most specifically the Chinese New Year when the whole city, as a matter of fact the whole of China, is on the move. The Lunar New Year is celebrated sometime in the latter part of January or early February. Of course, it would not hurt to immerse yourself more in Chinese culture and traditions when you partake of the celebrations of the New Year in Guangzhou.
The weather from May to September is hot and humid, and it rains almost every day with occasional typhoons visiting the city. If this is not to your liking, then visit during the other half of the year, when the weather is nice and the temperatures more bearable. Autumn is said to be the best time to visit the city because the gardens in the parks are all in bloom with clear skies and cool temperatures. The peak season for travel to the city is during summer, so autumn and winter will be your best bet.
If you can, travel during the week when everyone else is at work or school and the parks and other tourist attractions will have more room for your leisurely walk. Also you need to check whether the spots you will be visiting are open during holidays.
The Canton Fair which happens twice a year during spring (April) and autumn (October) is a major event for the city. Unless you are going to the Fair, you should avoid travelling to Guangzhou because the city gets crowded and hotel rooms become more expensive.
Just like the Fair, if you really need to travel to Guangzhou during peak season and holidays, bear in mind that you need to book your accommodations well in advance and realize that a lot of places in the city get crowded, making you wait longer to do many things.