It really depends on what the majority of your contacts are using/willing to use. The more you have on one program, the less work you'll have to do to convince other to join in.
Personally, I'm a big fan of WhatsApp. They just released a major update a couple of days ago, check it out OTA at whatsapp.com/ota . It is supposed to be $1.99/year, but the first year is a free trial, and you do need a BIS plan for it to work.
It is supposed to be $1.99/year, but the first year is a free trial
They extend the trial once you are close to ending the first year, they extended me for 6 months. We'll have to see if they keep doing that once i reach the end of it.
I've used PingChat, Kik, Live Profile, along with standard IM apps (via Trillian). The closest to BBM, in my experience, is Kik. It has been the most reliable for me. PingChat is also pretty good (now called Touch). I also tried XMS, but didn't know very many others using it (they were all using other apps, so XMS never got used).
I haven't used WhatsApp in quite a while. Last I remember, it used phone numbers to link people. A lot of the people I chat with don't have my cell number (by choice), so that limited who I could chat with. I prefer to use apps that use usernames.