Lag is the worst, input delay takes the cake, any world where you make a read, respond how you should, and opponent opens you up because yours came out late or not at all... its the worst.
example, i play my brother local and i beat him 100% of the time, i am undefeated against him Offline, then i can lose 50% of the time online, due to random screen jumps or inputdelay.
its madening to lose to someone you have played so much that thier moves are etchescetched into your brain, that mixup that is telegraphed, and you can't counter poke the D3 you block, and already input the punishing combo on hit that you knew would hit cuz you know they input another string after the D3, and yours comes out late and you lose for reading and inputing the proper punish.. i hate lag.