The question may seems to be easy one for some people but it is very tricky one for most of the people who may be planning to use any 
The question comes naturally that Which version should I buy? The answer is also simple i.e. Tally 9, The latest version from Tally India which is having most features and is updated regularly. The reasons are described here:-
Tally 9 is Latest and the Best
Let see it in this way. Which windows version are you running at present? The answer would be Windows XP or Vista, reason being its the latest and most of the features. Older version are not supported by your machine or they gets slow in operations. The same is the case with Tally also. Tally 9 is the latest and having most of the features. It has fast speed of processing and uses the latest tools and technology while working.
Previous Tally Version are Dead
See the Tally 7.2 End of Life document. You would find that just like windows 95 or windows 98 they are dead. No help is present on Company side, No tools are present to correct the data, technology on which Tally 7.2 has worked is obsolete. So if you have older version with you or you are arranging from some where, then be careful as are alone, as from company ‘s side these versions (any thing lower then Tally9 i.e. 7.2, 6.3, 5.4, 4.5 etc) are dead. Use these software versions at your own risk as no support on error or data lose would be available.
Updates are available only on Tally 9
Any software is successful for you only when it is updated on regular basis. For accounts this thing is very crutial as many govt. rules related to Taxation changes. This can be achive only when software is updated regularly. Tally 9 is the only version which is updated on regular basis. You would see the statutory version updation only in Tally 9. You would see these updates in almost every 15 days. I have not seen any other software company updating that much frequently.
Related reading:
Tally 7.2 Retirement
Getting started with Taly 9
Tally 9 is Latest and the Best
Let see it in this way. Which windows version are you running at present? The answer would be Windows XP or Vista, reason being its the latest and most of the features. Older version are not supported by your machine or they gets slow in operations. The same is the case with Tally also. Tally 9 is the latest and having most of the features. It has fast speed of processing and uses the latest tools and technology while working.
Previous Tally Version are Dead
See the Tally 7.2 End of Life document. You would find that just like windows 95 or windows 98 they are dead. No help is present on Company side, No tools are present to correct the data, technology on which Tally 7.2 has worked is obsolete. So if you have older version with you or you are arranging from some where, then be careful as are alone, as from company ‘s side these versions (any thing lower then Tally9 i.e. 7.2, 6.3, 5.4, 4.5 etc) are dead. Use these software versions at your own risk as no support on error or data lose would be available.
Updates are available only on Tally 9
Any software is successful for you only when it is updated on regular basis. For accounts this thing is very crutial as many govt. rules related to Taxation changes. This can be achive only when software is updated regularly. Tally 9 is the only version which is updated on regular basis. You would see the statutory version updation only in Tally 9. You would see these updates in almost every 15 days. I have not seen any other software company updating that much frequently.
Related reading:
Tally 7.2 Retirement
Getting started with Taly 9