There are many numbers of systems. Most affordable options include Acrylic Cementitious Waterproofing Coating, IPS Coba, etc. But it all depends on several factors such as:
How old is the slab?
What is the present status?
What is the life span expected?
What is the budget considered?
The best way to waterproof an old concrete roof/terrace depends upon the surface conditions and the age of the structure/roof.
The best option is Polyurethane Coating. The procedure involves surface preparation with wire brushing followed by cleaning and filling the exposed cracks with elastomeric material. Then prime the surface for better adhesion of the PU coating. Then apply the coating using brush/roller/spray in two coats. Ponding tests can be done to ensure zero water leakage. Although this system is a little expensive for achieving long-term results and durability this system can be installed.