Kongu districts - salem,erode, Coimbatore,nilagiri and Namakkal and Dindigul are greeny cities of Tamil Nadu Salem - yercard Erode - sathyamangalam forest, kodiveri dam in gobi Coimbatore - topslip Dindigul - Kodaikanal Nilagiri - Ooty, coonoor Namakkal - kolli hills TIRUPPUR - aanamalai hills
Bro, Nilgiris is totally hilly district covered by Green valleys, Tea estates, Botanical gardens, Tropical rain forest, rivers, dense forest etc. So I give Nilgiris in 1st. Kanyakumari is also beautiful and God's own district.
Madurai district has Sathiyar Dam, Kotladampatti waterfalls, beautiful palamedu town, scenic Vadipatti highway, Sathuragiri hills, Yanai Kesam waterfalls etc. Theni & Dindigul are old Madurai district.