I don't know about specific bars/clubs. It's not a bad idea though to sometimes go out on your own for a happy hour drink. I personally like to do this on occasion at the start of happy hour (during the first hour) when places aren't extremely loud - but, people are starting to come in. You never know when you meet someone who is there waiting for his friends to arrive - or maybe just there on his own too. I got a date once in one of those situations - guy was waiting for his friends, we chatted beforehand, he asked if he could give me his number or get mine - and we ended up meeting on another night for a date.

Where to go...? Kind of depends on you. Do you live or work in Manhattan? Seek out some bars that are convenient for you to get to. Live in Brooklyn or Queens? Find some neighborhood bars (or a neighborhood easy to get to from where you are) and give it a try. Honestly, you can always just walk in/walk out if you don't like it - but usually staying for one drink is harmless, and you might end up having a fun conversation with the bartender or with other people sitting at the bar. Yelp can be good for seeing what other people have to say about the "vibe" in different places. Do you like wine? I think winebars often attract a nice group of people if you're not looking for it to get too rowdy. But a fun divey bar can be worth seeking out too.