Doxybond-lb or penicillin. which one gives better result for curing stds like syphilis, chlamydia, gonorrhoea etc?

It really depends on which STD you need to treat. What works very well for one infection may not work at all well for another, and dosing and duration can also vary. If there is significant concern about exposure you could be given both of these antibiotics, pending more definitive testing, and that is not something that should be done casually or without medical supervision.