Season 7 is just around the corner and all players will need to start fresh. This guide includes tips for both solo and group leveling for freshly-created Seasonal characters. It doesn't have to do anything with powerleveling which will be possible once enough characters have reached Level 70.
Solo Leveling (1-70 with Massacre Bonus, takes anywhere from 2 - 3 hours depending on your class choice)
LEVELS 1 - 4
I. Create a game on Hard difficulty and adjust it to your gear and killing speed. While creating a new game, switch between Hard and Master difficulty.
II. Use Templar as your Follower.
III. Pick weapons of the other Followers (equip the best one and salvage the rest)
IV. Do a boss bounty. In solo on Hard difficulty, it's good to level up to 2 * 3 before you start the fight. Recommended bosses for shortest run times are Maghda, Zoltun Kulle. Keep in mind it absolutely needs to be a boss bounty otherwise you won't get chest with additional loot.
LEVELS 4 - 70
I. Grind Fields of Misery & Halls of Agony. You can kill the Butcher for additional loot. At Level 8 teleport to town and craft a two-handed axe at the Blacksmith.
II. Try to get the most out of the Massacre bonus by hitting mobs all the time while moving. You should practice long runs with continuous mob hitting if you're not comfortable with such gameplay.
III. Check vendors for rings (Level 7+ required) and amulets (Level 11+ required) and also go check the Blacksmith a few times to craft a weapon upgrade (every 10 levels or so).
Group Leveling (1-70, takes 2 hours and 15 - 30 minutes)
LEVELS 1 - 6 (5 - 6 minutes)
I. Create a game on Hard difficulty.
II. Pick Follower weapons (equip the best, salvage the rest).
III. Split Bounty Act I (kill the boss together then teleport to HoA 2 after the boss died and get Level 8).
LEVELS 7 - 8
I. Clear Halls of Agony Level 2 until you hit 8.
II. Teleport to town at Level 8 and craft a two-handed Axe at Blacksmith.
III. Check vendors for 2-4 damage ring affixes (Everybody should check out a different Act in the town to save precious time)
LEVELS 8 - 10
I. Do Nephalem Rifts. Right before the RG spawns a player should teleport to town and close the rift asap. During the 30-second rift closing window, check vendors for rings, amulets and Blacksmith for weapon upgrades.
II. At Level 10 a player should leave and run to the Skeleton King in Campaign then invite all players to get Leoric's Crown.
LEVELS 10 - 70
I. Run Nephalem Rifts and don't forget to craft a better two-hand every 5 - 10 levels.
I. Finish Chapter I - IV of Seasonal Journey to get your free set from Haedrig's Gift. Note that some chapter achievements can be completed even at lower levels.
I. You can craft two-handed swords at 15, 19, 23, 27, 32, 38, 44, 50, 55, 57, 61, 70; two-handed axes at 8, 16, 21, 29, 33, 37, 41, 50, 54, 58, 61, 70. Definitely craft a new weapon at 61.
II. Stay close to mobs when you level up, because you release an explosion dealing damage.
III. Get a Helmet with a socket as soon as possible, put ruby in it for Bonus XP. Leoric's Crown is ideal for this.
IV. Adjust difficulty depending on the speed you kill stuff. If you kill things fast, switch to harder difficulty and if you find yourself killing things too slow, don't be afraid to lower the difficulty.