All the actresses in tamil cinema are trying to occupy the number 1 spot that has been left open by actress Simrans marriage. Although Jyothika, Trisha and Asin have held that spot for some time, it keeps changing depending on who had the last biggest hit. So I dont give too much importance to who is number 1 and who is not. Let me just give a list of who I like to watch on screen.
1) Jyothika
Jo as she is called, has been acting in Tamil films for more than five years now, which is a big achievement. She has proved her acting talents in quite a few movies including Kaakha Kaakha, Dum Dum Dum and Kushi. She is interesting to watch because of her cute expressions and liveliness on the screen. She is also one of the few Indian actresses that can be good at comedy. The only thing that prevents her from being the next Simran is that she lacks glamour which is another way of saying she doesnt have a flat belly. But that doesnt take away from the fun of watching her on screen. She should be careful at selecting scripts though. However she is lucky to have an ally in director Goutham Menon, who makes it a point to include her in every single one of his movies.
2) Asin
Asin means pure. This girl from a rich business family in Kerala started of with a small role in a Malayalam film. She has come a long way since then, to become the reigning queen of Tamil cinema. She is very likeable on screen as well as pretty looking. Her performances in M Kumaran Son of Mahalakshmi and Ghajini were excellent. She also did well in other movies like Sivakasi and Maja where she only had short roles. It remains to be seen whether she will continue being at the top or another pretty girl will overthrow her.
3) Sadha
A beautiful face, enchanting eyes, charming smile and good body, more than makes up for her less than great acting skills. She excells in traditional girl roles as well as glamourous roles. Her looks in her debut movie Jayam gave many men sleepless nights. I reckon that the movie was a big hit mainly because of Sadha. Although she was in the big budget movie Anniyan later, she was not well appreciated by everybody because she was not that well picturized. They somehow used wrong makeup on her and put her in uncomfortable clothes. But if she could just improve her acting skills a little bit (not that it is bad right now), she will be a fine actress to watch.
4) Meera Jasmine
Well actually if you just look at her Tamil films this might come as a surprise. Her performances in Tamil films so far is not much to write home about, although I hear she did really well in some recent films. But in terms of just the acting, if given the right script and director, she can be absolutely phenomenal as shown by her perfomances in some malayalam films. She actually got the national award for the best actress for year 2003, which is a great achievement for such a young girl. Directors must use her acting talents properly and give her proper roles instead of the usual song-dance routine roles. What might prevent her from becoming the top actress though, might be the lack of glamour. She has  a great smile and beautiful face though.
5) Genelia/Sandhya
Its a tie between Genelia and Sandhya. Sandhya is the better actress of the two, however I havent really seen her in anything after her excellent performance in Kadhal.
To tell the truth, Genelia is not that good at acting. Some of her expressions are laughable or cringeworthy depending on whether you like her or not. I chose her because basically I like her more than everyone else that is left in the tamil film field and her cuteness makes her very very watchable despite her bad acting.
Who else are considered top in Tamil industry?
Trisha - favorite of many, but I dont like her too much.
Nayantara - favored by the masses, but I dont like her.
Namitha - B and C center favorite.
Sreya - Great looks, but acting skills, not too sure.
Sneha - nice smile but thats about it.
Upcoming actresses
Bhavana
is a really cute and talented young actress from Malayalam. I hope she will be accepted by tamil folks.
That concludes my review and its just my opinion and anyone is free to disagree. After all beauty and probably acting talent, lies in the eyes of the beholder.