I don't know anything about these groups (I'm more well-versed in Serbian than Bosnian politics, and even then...). After a cursory look none of them seem to be any sort of ML sect. Oštra Nula is a Western funded NGO, so YMMV on how good that is. Other than that all I can say is that a leftcom acquaintance from Serbia (who I would say has rather 'good politics') likes Basoc on Facebook.
Sorry I can't be of more help, I'm just an ignorant diasporan. Maybe I could ask if anyone knows more in this left-wing BCHS-speaking Facebook group I'm part of? They've recently had lots of discussions about the role of NGOs in the region so I imagine we could get some information out of them.
What will you be doing in Bosnia, by the way? How long are you there for?