which is the best program to draw characters?
There isn't a 'best' program. I couldn't live without photoshop, but that's because I use it for so much more. Paint could just as well create great sprites. It's a matter of preference. Well, and cash.
Try a few from this big list of mostly free paint programs (http://www.adventuregamestudio.co.uk/yabb/index.php?topic=5644.0).
SSH's character generator
Like they say, it's all a matter of taste. I swear by Paint Shop Pro 9 myself, it's powerful, user-friendly, and most importantly cheap as hell :). You can even pick up PSP 8 these days for around a tenner, and it's practically identical in its functionality. I'm also a bit of a rare case, in that I started off as a photoshop user, but migrated over to psp and never looked back.
For me MS-Paint, but as everyone has said, its a matter of taste.