i like gatorskins, but there's no "best" tire - it depends on how you use a bike. for daily use with commuting and crappy roads, though, yeah, gatorskins are very reliable.

i replace my rear tire when skidding has rendered it no longer round. the profile becomes severely flattened. sometimes i start to flat a whole bunch. for front tires - i usually put on a front and forget about it for a long time. i'm riding a tire that i had three years ago. i don't think i've ever destroyed a tire from riding it on a front wheel, but if i get a new pair, i cycle an old front to the rear. never the other way around - it's rear tires that get the most abuse - flattening and punctures. replacement time.

sometimes i'll destroy a tire before i get to this point, but that's infrequent.