Necro has a great summoner build that can get you pretty high in the GRs (80+, some say 100+).
Wave of Light Monk is insanely powerful, I jumped from Inna's T8/9 to T13 almost instantly.
Inna's is currently the gift for this season with the monk. All it really takes is Mystic Boots, which you can gamble at level 1, to become a good and powerful build.
How far back have you not played? Asking because you can do the "challenge rift" with any character and with completion, you'll get a nice gift basket. Contains 10 or 15 bounty act materials, ~450 bloodshards (use for mystic boots gambling), and pretty much enough gold and materials to get you to level 70 easily. Though, you can only get one per week, so choose wisely (seasons or nonseasonal, if seasonal make a level 1 character first).