Red Diamond Cafe is a hipster coffee bar in the Ladprao area. It is not quite accessible via public transport but it is definitely a very popular place amongst the locals.
They have a variety of self roasted coffee blends and the baristas are very friendly and are happy to guide you along with your choices.
The atmosphere in the cafe is superb and I really find it an awesome place to just chill and hang out with friends.
Address: 733 Pradit Manutham Road, Wang Thonglang, Bangkok 10310, Thailand
PS: I read that they now have an outlet over at Central World too!