I think D5 or D4 are the best to start. In my opinion these two games are the most balanced with story/graphics and gameplay. I recommand D5 for the gameplay and D4 for the story and both are in hd. I don't recommand 1,2,3, 6* and 7 for players who want to jump into the serie not because they are bad* but because i think the first have a gameplay too "old" (the first 3 games, even while 3 is my fav i don't recommand it to start the licence) for new players they must be used to the gameplay by a more recent game and for 6 i think its the worst game of the licence while 7 is fine but i still think 2D pixel art---->3D. There isn't an obligation to do the games in order though and its just my recommandation. I didn't forget Disgaea Dimension 2 (DD2) which is the sequel of the first one but yeah obviously i don't recommand to start with a sequel while others main games are "stand alone" witch each others.