Ok so basically you go through all of the first episode from season 1, 2 and 3 and decide which one winsm then the second episodes etc =D
So the pilot episode.
Ted meets Robin and Marshall and Lily get engaged. Ted tells Robin he loves her on their first date and blows it.
Season Two Premiere
The gang tries to help Marshall get over Lily, especially when he finds a credit card bill that leads to a hotel in New York City.
Season Three Premiere
When Robin shows up with a new boyfriend, Ted decides he is ready to hit the town with Barney. Enrique Iglesias guest stars as Gael, Robin's new lover and Mandy Moore guest stars as Amy, a tattooed party girl Ted hooks up with while out on the town. Marshall also calls Barney and shows him he has 55 days until his next slap.

Obviously I loved the first episode, it is where it all started but I just loved the second season premiere. It was actually the first episode I ever watched and I thought the whole Marshall punching a stranger thing was just so funny! I didn't think the season 3 premiere was as funny as it could have been (I also though Mandy Moore was crap to behonest!)

So to sum up the best episode number one of a season for me was the season two permiere =D

I hope all of that made sense, I'm very tired right now =D