In ODI I feel No 6 is the toughest position to bat.
We can take two kind of situations.
1st situation where a team scores 150/5 in 30 overs: In this situation the No 6 batsman comes under pressure where the team relies just on him and it is No 6 batsman who need to play sensibly without losing the wicket and also rotating the strike to keep the score board ticking, and he needs to change to aggressive mode after 45 overs. He needs to be calm at begining and agressive at the end.
2nd situation where a team scores 270/5 in 44 overs : The team is well settled in this situation and the role of No 6 batsman is to be agressive from the start. He has no time to get settled and need to strike the ball from his first ball also sometimes he has to play with tailenders which makes life difficult for them. If he fails to score runs in this situation then there will be trouble for his place in his team. He has to handle a immence pressure coming in this position.
As per my opinion, to open the innings is the toughest place to bat in test cricket. Batting conditions in test cricket are far more difficult than in limited overs cricket. Every wicket offers different challenges to the batsman. Openers are the first one to face those challenges without knowing anything about the conditions.Batsman following them get a fair idea about the wicket but openers have to adapt straightaway.Moreover, openers have to face the best fast bowlers of the opposition having the new ball making it really tough to bat.The bowlers too are highly energetic at the beginning of the innings offering their best.