Bangkok is lively throughout the year. The best time to visit Bangkok is really depending on personal preference.

Most people preferred to visit Bangkok between November to March, this is considered to be high season (Christmas and New Year time). 

The climate at this time is definitely not as hot as the rest of the year with temparature averaging about 25' celsius (77' Fahrenheit) to 30'celsius (86' Fahrenheit).

High season also means flights and hotels are more expensive, prior booking of these services is the preferred option for a visit during this time because hotels are likely to be booked out.

Bangkok has 3 main seasons

Summer - Usually between April to June
Rainy season - Between July to October
Winter - November to March

Please not that the above seasons in for Bangkok only. Weather in different parts of Thailand vary greatly

Today weather report

There's a reason to visit Bangkok in April because of the Songkran festival (Water throwing festival or Thai New Year). Read more about Songkran festival here.

It is a nice idea to consider the purpose of your visit to Bangkok. If you plan to enjoy the nightlife in Bangkok, you can certainly visit anytime of the year.

Here's a list of Public Holidays in Thailand.