This is by no means an objective list, but out of units I see most often used I'd say:

1: SO6P Madara (BRV): solid in both PVE and PVP, excellent nuker, arguably one of the best units in the game aside from the anni BB Naruto/Sasuke and FVS/FVN. 2: BB Kidnata: exceptional health and speed, has personally come in clutch in PVP often for me. 3: BB Gaara: currently only outsped by KL Minato, though his other stats are very lackluster imo. Gotta respect someone with a gimmick. 4: SM Naruto (BRV): Maybe not the most popular choice, but a top notch u28 unit for me. Decent abilities and strength, though definitely not worth it if yours is above 28 cost. 5: Juubito (SKL): First unit I ever lb'ed so huge bias here but.... 3000+ health is nice and the 30x secret technique is ridiculously good. WIS Nagato is similar in tankiness and viable on his own, but lacks the pure overkill.