First of all..... DON'T YOU NEED TO KNOW WHAT THE !@#$ IS IN THE HATCH?!!
My favorite was season 4, although from halfway thru season 3 to the end of season 4 is the best stretch overall. I voted season 3 as the worst, though, because the first half of that season was just incredibly boring. The writers have all but admitted they were stalling until they could set an end date for the show. They did that halfway thru season 3 and you can tell because that is when the series really takes off. I agree with UConn that season 1 is great for nostalgia, but it isn't the strongest overall.
However, I think you should quit now- it's just too much time and effort to watch a whole series. I joined in the summer b/w seasons 3 and 4, when they put the episodes on abc.com for free, and like you, I started from the pilot and marched right thru until I was caught up. I was hooked by season 1 episode 3, "Walkabout," when you found out about Locke's backstory, and I never looked back. So, if you aren't in love with the show by now, I just don't think it'll be worth your time. Find out what's in the hatch and leave it at that.