Which one is better maruti swift, i20, polo, fabia

Considering everything, Maruti Swift is the best car among all the cars listed by you, both in petrol and diesel versions. Polo and Fabia are only offered with 3 cylinder engines (apart from the 1.6 engine offered in both) which are much inferior to the Maruti K-Series 1.2l and the 1.3 DDIS engines offered in teh Swift. Most importantly, the Swift offers the best ownership experience and peace of mind because of Maruti's excellent after sales service network and prompt customer service. The i10 Petrol has more features but is more expensive and the i20 diesel is simply way too expensive to make a good case for itself.