Generally any block will be a fun enough experience to buy packs of and build decks out of what you get, but be forewarned that the current block, Khans of Tarkir, has the fun but complex mechanic morph, which may be hard to understand for a new player.
However, what I recommend doing is getting a Deckbuilder's Toolkit to get a decently sized collection of cards, and then after you try to build decks out of what you have, you can then move on to buying individual cards to fill out what you think is missing. While it's fun to open up booster packs, generally, it's recommended that you try to do something with booster packs you get, such as play in a limited format at a local store, since it's otherwise more cost effective to get individual cards.