All the BBA programs are run on semester system, which means that you'll have to appear board exams in every six month during the program.
Kathmandu University (KU)
The BBA of Kathmandu University is recognized to be one of the best and well-managed programs. KU provides BBA degree through its affiliated colleges like Kathmandu College of Management, Little Angel's College of Management and Nepal College of Management; as well as its constituent campus, Kathmandu University School of Management. You can learn more about it here.
Tribhuwan University (TU)
The BBA of TU is renowned for the brand name of the university. Especially in Kathmandu, the craze for the BBA of TU is high. There are 17 colleges in Kathmandu and one more in Pokhara which can provide you the BBA degree of this university. The students' performance is evaluated both internally and externally with the weightage of 40% and 60 % for the internal and external assessment respectively. You can learn more about it at http://www.fomtu.edu.np/programs/bba.htm
Pokhara University (PU)
The BBA of Pokhara University has the course curriculum of international standard. I think Tribhuvan University's curriculum isn't much different either. However, that's a perception I have heard about. I am currently studying the BBA of PU and I have found its curriculum to be very good. The evaluation criterion of PU's BBA comprises of 50% internal and 50% external assessment. To learn more about it, visit http://pu.edu.np/bba.php and http://pu.edu.np/bbaps.php
Purbanchal University (PurbU)
The BBA of Purbanchal University is the one I would least recommend. The main demerit of the BBA of Purbanchal University is that its exams aren't held timely. In the semester system education, nothing is more painful than the delay of examination. You already have to study four years for a bachelor level. And if time goes beyond this limit, it will be really nauseating. Even on the curriculum front, other universities are ahead of it. So, I don't recommend you to register for the BBA of Purbanchal University. What is worse that its website has expired and I can't provide you the link to learn more about its BBA program.
In short, it is not the university that is going to study for you. It is the job of students themselves to study well. However, if the university doesn't regulate its examinations well and updates its curriculum regularly to reflect the latest developments in different subjects then the student's effort will either be useless or will go in the wrong direction. Therefore, it's vital to select good university even if you aren't planning to study well. Now, if you have enough money I would recommend you the BBA of Kathmandu University. If you can't go there, I would recommend good private colleges which run TU's or PU's BBA program. Since, education is a onetime investment which provides a lifetime return, I recommend you to study in good colleges, which teach you skills, by paying higher fees rather than to study in a bad college by paying a small fee. If you get a cheap college which provides quality education, then consider yourself lucky!