You are in luck!
The fluffy old lists are now the Competitive lists. Wych cults are now the best, Kabal #2
and Coven #3
Basically everything in the codex is good to amazing.
"Worst" units are basically the RWJF, Beast Packs, Talos (completely my opinion). These units are the "worst" in the DE codex, but many armies would kill to be able to field them XD
Voidraven bombers are very competitive. Wyches and hellions/reavers are very competitive. Transports are game breakingly competitive.
Its honestly hard to build a BAD list. Im going to a crusade game tomorrow (1k points), it was actually difficult to build something that WASNT competitive XD.
Start off small, generally a patrol of your favourite sub-faction, then build up from there. The game is very different these days to 8th (unless you played ITC, then its basically an inferior copy).
Welcome back, good luck and happy raiding!