Yeah, time travel stories rock.

Here are the truly superb Star Trek time travel episodes:

Tos :

All Our Yesterdays
City On The Edge Of Forever


Next Gen :

Time's Arrow 1 & 2
All Good Things


DS-9 :

Past Tense pts 1 & 2
Little Green Men

Voyager :

Future's End pts 1 & 2
Year Of Hell

Enterprise :

Storm Front pts 1 & 2

Let's not forget Trek movies :

First Contact

"I offer you a Vulcan prayer, Mr Suder. May your

death bring you the peace you never found in

life." - Tuvok.