What to do before MS Software Engineering in Pakistan, BS Computer Science or BS Software Engineering? Which is best combination?

I think it doesn't matter. There won't be much difference between CS and SE. If you want an MS in Software Engineering, try to go for a degree that has more focus on software related courses. Many universities have their own naming convention, so its more important to see what courses they include. E.g. GIKI offers Computer System Engineering and Computer Software Engineering. Software Engineering would have courses biases towards programming, design patterns, web etc while System Engineering would be focused on the hardward side, electronics etc, although most of the courses would be common to both.

Actually both are same, they teach you approx same subjects.
You can select SE elective courses if you want to specialize in SE in BS.
Majority university in Karachi i know, offers you BS-CS or CE. Computer Engineering is more into Hardware.
SO BS(CS) or BS(SE) is almost same. it depends on you which elective courses you choose while graduating

I agree with @Usama Hafeez, but as your next goal is MS Software Engineering then BS Software Engineering is the best combination. It will be more helpful for you as you can study in depth SE.

i think both choice is best both are the same and database oops data-structure language but after this u do ms depend on Ur skill

CS give you the way to write a code in order the handle or to solve the problem and SE teaches you the tactics how to make better software product in all aspect. I think it is better to do BS(CS) and choose the elective subjects that is related to Software Engineering and then enter into MS(SE).