Nowadays each email service provider is trying to seperate itself from the rest by providing services which are unique.
Lets start with Yahoo Mail which upgraded last year to a swooping 250MB storage capacity, this all I find interesting in Yahoo Mail the rest is plain garbage, advert and spam.
GMAIL providing 1GB mail now provides POP3 access(maybe this was not the case at the time of your writing), mail forwarding and filtering functionalities, on top of that it allows you to search your mail box as if you were searching for something on google, which is really cool. Still in beta this baby rocks!
Rediffmail in my opinion is really too advert centered and lacks the maturity of a mail service. There is something in rediffmail which makes you think that its not professional enough as to provide something really genuine.
Dont forget http://www.fastmail.fm which provides for 10MB and IMAP support.
Hotmail has upgraded 2MB accounts to 25 MB for some countries later in the year these will be upgraded to 250MB. But as it is doing this upgrade on a per country basis it is expected that it will take a long time before everyone is satisfied. Hotmail now comes with a tight integration with MSN messenger, which I find really cool.
To summarize my opinion would be the best mail service really depends on your needs, whether you need the latest technology, the biggest mail box size, the most stable-secure mail service, . its just comes down to finding what suits you best and there is a lot to choose from.
ciao,
Javed