SSJ4 Vegito

I don't know about placing SSJ Trunks & Goten around Perfect Cell it seems to be overdoing it a little bit. Remember during Goten's Sparring match with Gohan, Gohan did not have a MSSJ aura similar to how Goku didn't have one during his "Warm up" against Cell. So I think Gohan most likely didn't use the Full extent of his power to make his match with Goten more fair. Also it might explain why Gohan's hair as a SSJ changes to his SSJ2-like hair style (Just no lighting) when he is angered during the fight between Videl and Spopovich and his fight with Dabura (Where he had a MSSJ aura). So since all his moves where blocked by someone weaker than Perfect Cell (Suppressed) I think Trunks & Goten fall around Trunks & Vegeta in the Cell Games.
As for how strong I think Piccolo is, I'd say he is around Suppressed Perfect Cell considering the fact that he trained after Frieza (And unaware of any sort of threat) we can assume Piccolo always trains. So with 7 years to power up I'd say & Pre-Budokai Training I estimate him around SSJ 3rd grade Trunks in his fight with Cell, and just the fact that Broly was able to take in most of their attacks when they were SSJ. I'd give Broly a power advantage but Piccolo is a better fighter of course that's my opinion. I think either one of them could win.