I assume by worst, you mean nastiest or most uncomfortable? Because if you mean least entertaining then a good portion of each series when the HMs are sleeping or sunbathing would be mentioned.
Hard to narrow it down to two single ones, but I'll provide a list:
Best
- Jade vs. Adele's argument over a verucca, purely for Alex's reaction in the background - "Do something for me!"
- The Jade vs. Kate/Jonny vs. Alex argument near the end of the series - spawned some classic BB quotes such as "I've been up for nomination three fookin' times!" and "Lick the arse, Jade!"
- Fight Night - Perhaps uncomforable viewing, but when I watched it, I was thinking, wow! This confirmed that BB5 had taken Big Brother to a whole new level like I hadn't seen before. I also think that ironically, compared to certain other heated arguments that had happened on Big Brother, there wasn't as much of a nasty underbelly, it was just a group of drunken idiots being aggressive. Overall I think BB5 was actually one of the least nasty series'.
- Victor in the diary room rapping about how much of a badman he is, before asking for some more bread and milk.

- Victor's eviction, genuinely one of the greatest. A lot of HMs in the past have been self-conscious about being booed, but he seemed to thrive on it, and kept encouraging the crowd. Fair play to him.

- "Yeah...Brackie"
- Kemal's "someone got wet" comment. :O
- Science calmly saying "Eurgh, plaque attack, brush your teeth" when Maxwell was shouting in his face over the wine.
- Maxwell's eviction! He looked like he wanted to cry!
- Sezer's reaction to his name being called out too.
- Aisleyne standing up to Grace over the water being thrown over Susie, and the ensuing argument in the bedroom, culminating in Richard mimicking an aggressive Lisa.
- Aisleyne destroying Lea
- The prison task
- Leo Sayer's breakdown, first the "thumb/finger" slip up, and then his escape - "Don't touch me you bastard!"

- Darnell during Spitgate, what a hero
- Mario's entire tenure during BB9

- Mikey throwing chickpeas all over the kitchen

- Coolio and Tina's argument - "Hungry ass!"
- Freddie's Shakespeare impression

- Bananagate, I hadn't laughed so hard at BB for so long, but I was in hysterics. Harry was an absolute star that year.
Worst:
- Seeing Jonny try to take advantage of a very clearly intoxicated Kate

- Lesley's bullying of Sam
- Maxwell basically trying to punch Kemal in the same incident that Kemal made the "somebody got wet" comment.
- The ostracisation of Science
- Shahbazgate. One of the only two occasions where I genuinely felt infuriated watching Big Brother
- Grace throwing the water over Susie, and Mikey telling Aisleyne to "shut the **** up!"
- Spitgate, of course. People say that Mohamed was a credit that night for not retaliating against Dennis, but he would have been a legend in my eyes if he decked the vile rat. I wouldn't have minded seeing Dale and Stu having one laid on them either.
- The treatment of Freddie
- The treatment of Deana after she threw up during the eating challenge. Horrible bunch of bastards.
- And, of course, the epilator comment