I can't really rank the seasons, having only seen the show once so far. I think season 1 or 3 is probably my favorite, and 5 is the least, but beyond that, I dunno. Hard to tell how the surprises and character developments and flashbacks hold up a second time around. I'll probably marathon it all again later this year.
I'd like season 2 better if more of the cast had survived it. I'd be really curious to see how different the series would be if Eko had lived since his belief in something else was clearly going to strongly counterpoint Locke's belief in the island (which turned out to be the smoke monster.) I could see the final confrontation having been Eko/MIB Locke. I know Jack is the obvious main character, but thematically he wasn't really right for that role, and had to change a LOT to fill the spot that others did better.
I think it'd be really interesting to see the series in basically chronological order. I know that's impossible due to the sheer ammount of jumping around involved, how long the series was, and that the episode asides are usually tied thematically to the episode they're in, (and also would want individual character stories to go linear probably) but… would be interesting, I think. Mostly you'd need to split season 4 apart, I suppose.