Depends on your priorities.
Santro: If a good suspension is what you're looking for along with good ground clearance, santro is the better choice. The engine performs better than Cultus or Alto and the air conditioning is also better than the other two. The chassis is also stronger and the rides much more comfortable than the other two. However, not all mechanics are familiar with santro and it is important to have one mechanic (who is familiar with santro) to take care of it. Parts are hard to find and relatively expensive as compared to Suzuki parts. But they will not be going obsolete at all. I assure you. But still, you can't find parts in every market. I drive a 2004 santro because I preferred it over my 2008 alto.the santro seats are much more comfortable for long drives than an alto . No resale value. You will get the same amount roughly when selling as you will pay to get the car right now.
Cultus: seats are soft. More leg space for passengers as compared to alto. 1000cc engine runs well. The AC is better than alto. If you're going for an old Cultus , the mileage will probably be bad, but the car will have good acceleration and torque as compared to newer models. It is a low ride as compared to alto and santro, you will feel like you are sitting on the ground(but that's just my opinion). Parts are readily available in the market. Body will most likely not be in good condition after all these years.
Alto: cute car. Everything is average. For short drives within the city in traffic it is a good car. Good mileage. If you run it on cng or with the AC on, the car engine will start begging for mercy .parts are available everywhere. Good resale value. Easy maintenance. Breaks are not very great but they're okay. Suspension is ok as well. It is not as low as a Cultus , so that's a plus when you're driving in traffic jams. Good luck!
