"The Maternal Congruence" has the return of Dr. Beverly Hofstadter (Leonard's mother) and includes some brilliant jokes (her ongoing take-down of Raj and Howard a particular highlight), but also demonstrates the show's stretching for material - particularly Penny getting Beverly drunk and her resulting behaviour.
"The Precious Fragmentation" shows the boys in possession of one of the actual prop rings from "The Lord of the Rings" series, and the amount of turmoil such an item of geek chic produces among them: Penny punching Sheldon when he tries to steal it from her when she's sleeping was a moment of comedic genius - followed by another as Leonard, having woken and realized what's happened, says "that's my girl." As the series started to stretch for material, this episode felt truer to the characters than most.
"The Pants Alternative" includes an improbable but hysterically funny set piece with Sheldon drunk and singing Tom Lehrer, taking off his pants while discussing topology, and then having his memories of the evening restored by YouTube.
"The Adhesive Duck Deficiency" is brilliantly funny in places (Leonard, Raj, and Howard's stoned confessions) and a prime example of the show stretching for material (the hospital trip).
This season included the introduction of Bernadette Rostenkowski (Melissa Rauch), who appears in several episodes and becomes Howard's girlfriend. Their bonding over their over-protective mothers ("The Creepy Candy Coating Corollary") is clever and hilarious, but I failed to find her ongoing interest in Howard particularly convincing. In the last episode of the year we're also introduced to Amy Farrah Fowler (Mayim Bialik), who will become Sheldon's love interest.
At this point, I think I've seen as much of the series as I really need to - from now on its diminishing returns as the situations become more absurd and humiliating to milk more humour out of the series (although the cameos are just ramping up - they had a lot in later years, and many of them were wonderful).