I've used the g400s forever and it recently died. I went with the g403, i recommend it.

zowie fk is a tftv favorite
or
g502/etc

most (not all) of the logitech mice people are going to recommend you have the same bretty gud sensor, so there really isnt a world of difference between them in that category

I've been using the razer deathadder 2013 for almost five months now, it feels very comfortable for the clawgrip style I play in, and was night/day from a $3 non-gaming mouse I used before. I'm not really an expert on mice but I can say that the deathadder feels very comfortable for long gaming sessions and has yet to show any signs of wear and tear. If you're near a store that lets you test out mice before you buy them I would look to do that to make sure it fits your hand the way you want to.

seconding the g403; its got one of those funky braided cables which i thought looked dumb but the cable feels much sturdier, plus its detachable/replaceable

only niggle is there's just 1 dpi button unlike the g400s, so you have to cycle through them all if you want lower dpi

the g502 is probably the best bang-for-buck mouse i can think of off the top of my head

you can also do this with it which is cool

as a g502 user for almost 2 years now, I strongly recommend you not to go anywhere near this mice. specialy if you're in us and have access to mice with 3360/66 sensor that dont look like fucking alien spaceships and are lighter, hence easier for user to adjust to them. Like g403.

g403/gpro

My way old g400 died a year ago as well, it was seriously the most comfortable mouse ever. Now I'm using zowie fk1, which is pretty neat too. It took me like a week or 2 to get used to it though as it's not as steep in the back as g400.

g403 is so good and it doesn't look like it can second as a star wars speeder bike.