This is the Top 10 best performances according to a very popular Glee fansite called http://www.wetpaint.com/glee. It's an awesome website that I get all my knowledge on up and coming Glee stuff. I have added in my personal opinion of the song and a link to the video on youtube.
10. Without You - Rachel (Episode 10, Yes/No): This song is amazing. It was a pretty slow song but that's the whole thing about Rachel; she's more of the kind of person who's sing slower songs because they fit her voice perfectly.
9. Let It Snow - Kurt & Blaine (Episode 9, Extraordinary Merry Christmas): This and 'Do They Know It's Christmas' were probably the only good songs from this episode. 'Let It Snow' was awesome but it doesn't beat Kurt and Blaine's duet in season 2's christmas episode of 'Baby It's Cold Outside'.
8. Paradise By The Dashboard Light - New Directions (Episode 21, Nationals): I love this song! It's one of the coolest songs they performed at Nationals (well, they've only been to Nationals twice). This was just an amazing group number.
7. Not The Boy Next Door - Kurt (Episode 18, Choke): Kurt made this song awesome. I never actually heard this song ever before Glee covered (like most other songs Glee covers).
6. We Are Young - New Directions (Episode 8, Hold Onto Sixteen): A perfect song for them to sing after Sectionals and when they all unite. I had never heard this song before Glee covered it but about a month later the Glee version released in America, the original became one of the most popular songs in New Zealand.
5. Perfect - Kurt & Blaine (Episode 7, I Kissed A Girl): Just perfect (see what I did there). Another Klaine duet is on the top 10 since Kurt and Blaine's voices go just perfect (I'm getting extremely cheesey here) together.
4. Run The World (Girls) - Brittany & Santana (Episode 3, Asian F): Brittany didn't get a lot of songs this season (most them with Santana when she did sing) but she is one of the coolest singers on Glee and she's an amazing dancer.
3. Roots Before Branches - Rachel & Finn (Episode 22, Goodbye): This is the song I always cry in. Two reasons: 1. It's the song where Finn 'sets Rachel free' and 2. Lea Michele (who plays Rachel) always makes me cry when she sings emotional songs.
2. Rumour Has It/Someone Like You - The Troubletones (Episode 6, Mash-Off): A great mash-up which marks the 300th Glee performance (now they've done more than 300). I think that Santana and Mercedes have the best voices on Glee and they fit perfectly together.
1. Smooth Criminal - Santana & Sebastian (Episode 11, Michael): Best Glee song ever! (well, maybe not as good as Finn & Rachel's amazing duet of 'Faithfully' back in Season 1). Naya Rivera (who plays Santana) and Grant Gustin (who plays Sebastian) both have these awesome voices so them singing together was just perfect. Also, whenever this song is playing on my ipod, I always listening to it.
Honourable Mentitions in my opinion:
-Somebody That I Used To Know - Blaine & Cooper (Episode 15, Big Brother) > http://youtu.be/0cay2dnuhcs
-Shake It Out - Santana, Tina & Mercedes (Episode 18, Choke) > http://youtu.be/fkOiIsEpehc
-Boogie Shoes - Wade/Unique (Episode 16, Saturday Night Glee-ver) > http://youtu.be/xRFLOGOvSWg
-We Found Love - Rachel & Santana (Episode 10, Yes/No) > http://youtu.be/PCDAxeuJn6I
-Stereo Hearts - Joe, Sam, & Mercedes (Episode 14, Heart) > http://youtu.be/mbUeKyzAiqU
-La Isla Bonita - David Martinez & Santana (Episode 12, Spainsh Teacher) > http://youtu.be/1W7AoJ39Gu4
-What Doesn't Kill You (Stronger) - Mercedes, Santana & Brittany (Episode 14, On My Way) > http://youtu.be/j8OpA_f4DtM
-How Will I Know - Rachel, Mercedes, Santana & Kurt (Episode 17, Dance With Somebody) > http://youtu.be/jkhT_W0CR6w
-Cough Syrup - Blaine (Episode 14, On My Way) > http://youtu.be/JdfuhAoA_9M
-Hit Me With Your Best Shot/One Way Or Another - Finn & Santana (Episode 6, Mash-Off) > http://youtu.be/Z5SkXW3ZkYQ
Might as well say the Top 5 worst performances of Season Three while I'm at it.
5. Red Solo Cup - Sam (Episode 8, Hold Onto Sixteen): It was a good way for Sam to return to Glee but it wasn't as amazing as every other performance this season (or ones that Sam has sung).
4. Stand - Sebastian (Episode 14, On My Way): Grant Gustin is talented. He has a great voice but this performance wasn't awesome. It feels so strange seeing someone other than Blaine led the Warblers.
3. Dinosaur - Brittany (Episode 19, Prom-asaurus): Brittany is awesome but the performance was really weird (especially the dinosaur headed cheerios).
2. A Little Less Conversation - Mr Will Schuester (Episode 12, The Spainish Teacher): This episode featured a bit of a spainsh song sing-off between Will and David Martinez (Ricky Martin). The winner (by far) was David (who performed with Santana and sung 'La Isla Bonita').
1. The Rain In Spain - Puck & Finn (Episode 18, Choke): This song was terrible, that's all I can say about it.
I can't wait until Season 4 premieres and it has already been announced they are doing another Britney Spears tribute (after the sucess of Season 2's Britney Spears tribute). It's going to be an interesting season.