honestly, there are so many multi platform messengers out there now that you should have a field day testing them out....I always go back to whatsapp because it works the best and has the best battery life out of all of them....liveprofile looks good as does kik but both are battery hogs...
I like Skype as a basic IM client for everything (iPad, computer, BB, iPhone, Android etc etc).
I also like Kik, but nobody I know uses it anymore. I don't think any of my friends use other IM clients... just texting and facebook message (LOVE the BB FB messenger app... connects/syncs with iPhone/Android/Computer perfectly).
I don't know of any cross-platform IM clients (other than skype) that have everything BBM offers, I have yet to find one where you can create groups.
Not the yahoo/aim/google messenger. they kill the battery on the blackberry. (havent tried it on other platforms, but I would think the same)
Kik. No one uses it, so I haven't had much experience.
Live profile looks allot like BBM with the "R" and "D", but this app is very limited on the Blackberry. (you cant sent pictures/videos). It also crashes allot on BB. it works better on other platforms
you mention not Whatsapp, but..
Whatsapp is probably the best of the bunch, since its the most popular and it requires all users to have the app installed. Disadvantages are: no "D" and "R". App crashes some times on the 3 platforms(iOS Android and BB). Its only free on BB and Android. iOS users will have to pay $1 for the App. Every now and then it fails to push the notification of new message until you open the app on all 3 platforms.
BBM is the best for what it does, with iMessage close second, but they are both only within the same platform.
I forgot about WhatsApp! I use that too. Whats nice about WhatsApp is that it uses your number, so everyone on your phone contact list who has the app is automatically added. Makes it very easy to operate and communicate with.
My family is pretty evenly split between iPhone and BB, we use the group-chat feature in Whatsapp all the time; I think it is pretty much the next best thing to bbm.
Have to admit, I hated the older version of whatsapp. I didn't know the new ver was out. Looks awesome, works great! Minimal battery drain. Whatsapp ftw!