ok lets split this by class, but keep in mind all of this is subjective as personal taste is an important factor to consider.
Monk: Wave of Light Sunwuko, crazy strong, crazy fast, crazy efficient for all game modes. Can become boring due to ease of play but trust me, its really strong.
Demon Hunter: Mulstihot build, same things as above. Its fun feeling OP with it, but if you want more fun than efficient gameplay i'd say Shadow Impale and Marauder are still viable choices.
Barbarian: Raekor Charge is efficient but awkward to play. Whirlwinf requires godly gear and para to be good. Earthquake is strong enough to plow through lower GRs while feeling the oomph behind your skills, making it a fun build to play.
Necro: Properly built Pestilence is OP and fun to play and watch. Nuff said, everything else is either underpowered or boring.
Witch Doctor: Seeing you are a fan of pets, helltooth gargs are your best bet, built properly makes your pets scary as hell.
Wizard: So many builds here to try if you are going for lower GRs and not push. DMO frozen orb, TalRasha meteors, Lightning Archon, Talrasha supernova, lots of viable fun builds, i prefer TalRasha fire meteors for fun(star pact is better but a chore to play).