As far as Tamil Cinema's climax portions were concerened we can't say like only one film has 'The Best Tamil Movie Climax Ever'.
So keeping things aside and to answer this question, I would say, THE BEST CLIMAX PORTION goes to AARANYA KAANDAM!!
The Best Tamil Movie Climax
1. "Jigarthanda" . The climax is amazing. everything is perfect.
2. "Maalai nerathu mayakam", written by selvaragavan. Amazing, The whole movie is underrated. The climax touches you. Please wait and watch the scene that come after the titles.
3. All bala films. Esp. the one that touched me in a personel level was "PARADESI". If you have a solid heart, still you would cry for the plight of his family.
4. PIZZA.The climax of "PIZZA" is quite unexpected, not in a lofetime did i expect it to end like that.
5. "MANGATHA". I wonder why people havent mentioned "mangatha" here. The climax of mangatha was quite new to the entire tamil industry.
6. "Kadhai thiraikadai vasanam iyakam", is yet another movie that had a new ending in Tamil Movies
, The climax was really good.
7. Azagai irukirai bayamai irukirathu . This movie is not a movie that is not liked by many.
8. The twist in the movie "IYARKAI", that is really worth a mention.
9. "Paruthiveeran", has a different ending.
10. The movie, "ISAI", by S.J. Surya has a good climax.
11. The climax of movie "lucia"/ "Ennakul Oruvan", is different and teaches an important message "the life you are living, is someones dream".
12. 7G Rainbow Colony and Kadhal konden also deserve a place here. Most of selvas films.Best Climax From Kamal Movies
Being a huge Kamal fan, my answer may be biased with the mention of just kamal movies. Despite a lot many more, I want to mention these.
Here are my favourite ones:
1. Nayakan: The beauty about this climax is that Mani Ratnam and Kamal Hassan met Varadaraja Mudaliar and asked him how he would foresee the ending. That lead to the final scene of the Tamil movies
2. Punnagai Mannan: Back to square one would be apt to describe this. Starting and ending at the same waterfall with some drama in the middle and exquisite music makes this one as one of my favourite.
3. Kuruthi Punal: As they say, the show must go on. Only an actor like Kamal could conceive such a screenplay with minimum characters in the climax in an era when climax meant a lot of the film's supporting cast came together and completed it.
4. Uttama Villain: I don't know if it is a climax or anti-climax. He dies but doesn't die. Watch it yourself to appreciate it and then the verses that follow it.
5. Paapanasam: The latest addition to the list is this. Agreed this is a remake, but be it the original or the remake, the climax was fantastic. I was as awed by it as I was by seeing Shawshank Redemption. The beauty in this one was everyone knew that the protagonist would save himself but how he would do it was the key and the screenplay just nailed it.