>>50667109XYORAS. They're the one games that got dex completion right by having them all be available between the two games and in ORAS the DexNav makes completing each route fun and engaging. Gen 5 is a close 2nd because seeing each Pokemon in that style is beautiful and the games themselves are just fun to play.
The worst / least rewarding are Gen 2, 7, and 8 because of the lack of incentive or tedium.
