In your opinion, what are the best episodes respectively featuring one of the principal characters as the main protagonist? Oh, and why? 
Captain Jean-Luc Picard
For me, it's a toss between Tapestry and The Inner Light, two outstanding episodes. Tapestry provides an interesting analysis of Picard's character, while The Inner Light is just a wonderful story beautifully told. Another favorite is The Perfect Mate, where I was pretty impressed by how well Stewart portrayed his character's affection for Kamala.
Commander William T. Riker
With Riker being my favorite character this is pretty hard to nail down to just one episode. Favorites include Second Chances, Peak Performance, The Pegasus, Future Imperfect, A Matter of Perspective, Frame of Mind, The Outcast and of course The Best of Both Worlds (which in many ways can be seen as a 'Riker episode'). Out of these The Outcast is probably the episode which utilizes the character in the most interesting way. Frame of Mind, though, is brilliant, too. I just can't make up my mind ...
Lt. Cmdr. Geordi La Forge
Geordi I love almost as much as I love Riker. However, it's much easier for me to name my favorite 'Geordi episode': I like the situation which Geordi has to face in Galaxy's Child and think it made his character one of the most human in the series.
Lt. Tasha Yar
My favorite 'Tasha episode' would have to be Yesterday's Enterprise, which gave her character a much better send-off than Skin of Evil.
Lt. Worf
Hm, I never really liked Worf that much, but I loved him in Parallels. Dramatically, though, I think Ethics might be the better episode.
Doctor Beverly Crusher
Crusher was never one of my favorites either, but I mostly liked her in the episodes featuring her as the main protagonist. I loved her in Cause and Effect, which practically is a 'Crusher episode', if you think about it. Remember Me was cool, too. And I liked her in Sub Rosa. Yeah, I'm not kidding.
Counselor Deanna Troi
I know I'm kind of cheating here, but I loved Troi in Power Play, which gave Marina Sirtis the chance to show off a lot of her acting abilities. Likewise, Face of the Enemy provided a good opportunity to see Troi outside of the usual 'Captain, I sense ...' situations.
Lt. Commander Data
Data was the star of many episodes. I loved him in episodes like Data's Day and Birthright, Part I. But I think his strongest episodes were The Offspring and In Theory, because they actually made me feel sorry for a being incapable of having emotions.

Captain Jean-Luc Picard
For me, it's a toss between Tapestry and The Inner Light, two outstanding episodes. Tapestry provides an interesting analysis of Picard's character, while The Inner Light is just a wonderful story beautifully told. Another favorite is The Perfect Mate, where I was pretty impressed by how well Stewart portrayed his character's affection for Kamala.
Commander William T. Riker
With Riker being my favorite character this is pretty hard to nail down to just one episode. Favorites include Second Chances, Peak Performance, The Pegasus, Future Imperfect, A Matter of Perspective, Frame of Mind, The Outcast and of course The Best of Both Worlds (which in many ways can be seen as a 'Riker episode'). Out of these The Outcast is probably the episode which utilizes the character in the most interesting way. Frame of Mind, though, is brilliant, too. I just can't make up my mind ...
Lt. Cmdr. Geordi La Forge
Geordi I love almost as much as I love Riker. However, it's much easier for me to name my favorite 'Geordi episode': I like the situation which Geordi has to face in Galaxy's Child and think it made his character one of the most human in the series.
Lt. Tasha Yar
My favorite 'Tasha episode' would have to be Yesterday's Enterprise, which gave her character a much better send-off than Skin of Evil.
Lt. Worf
Hm, I never really liked Worf that much, but I loved him in Parallels. Dramatically, though, I think Ethics might be the better episode.
Doctor Beverly Crusher
Crusher was never one of my favorites either, but I mostly liked her in the episodes featuring her as the main protagonist. I loved her in Cause and Effect, which practically is a 'Crusher episode', if you think about it. Remember Me was cool, too. And I liked her in Sub Rosa. Yeah, I'm not kidding.
Counselor Deanna Troi
I know I'm kind of cheating here, but I loved Troi in Power Play, which gave Marina Sirtis the chance to show off a lot of her acting abilities. Likewise, Face of the Enemy provided a good opportunity to see Troi outside of the usual 'Captain, I sense ...' situations.
Lt. Commander Data
Data was the star of many episodes. I loved him in episodes like Data's Day and Birthright, Part I. But I think his strongest episodes were The Offspring and In Theory, because they actually made me feel sorry for a being incapable of having emotions.