In Anaheim where Reggie Sanders threw a bucket of gum onto the field after getting kicked out of the game, and Ruben Rivera hit a go ahead / game winning HR over the LF fence in the top of the 9th.  I guess that's not historic, but it will always stick out to me for those two reasons.

Although I've tried to erase the memory, I was also at Burnitt's no hitter against us where he - ugh - walked 9 and hit a batter.

but the game that sticks out so much in my mind as so memorable to me that year was the night they clinched the NLWEST. it was on sept 12th against the dodgers on a saturday night. we fell down 0 to 7 in the fifth, got a few runs in the bottom of the fifth and then in the bottom of the 6th the magic happened. the fans went absolutly nuts, as loud as any playoff game i had been to. they basically walked 5 i think in the inning and sheets got hit by a pitch, and we came back to win 8-7 and clinch the west. it was a crazy game the crowd really had an infuence and was getting in the pitchers heads.

heres the box score

http://www.baseball-reference.com/boxes/SDN/SDN199809120.shtml
 

2005. First ever postseason game at PETCO
2006. Trevor breaking the saves record

Why do we remember certain games?  This is one that I do.  I was there with a friend (we were in HS), sitting in 3B Field (although we only paid for GA!)  

2 out, none on, down by 2 in the bottom of ninth.  The rally was kept going by an E6 to Johnnie LeMaster.  I'll never forget it --- grounder right through his legs!  Also, this was a game during an umps strike, so they had replacements.

Other (more important games):
Last at the Murph
First at Petco
The playoff game in '96 (loss)
Trevor's tie and break saves
CY's near no-hitter
I also saw Aaron it #'s 706 & 707 (or there about) in Sept '73
And AJ Burnett's no-no (ugh!)

-last game at the Q
-Tony's last game, which was also when Rickey got his 3,000th hit
-the 16 inning game vs Pittsburgh in 1996 (or 98?)
-the game where Klesko hit the walk-off homer against Arizona in the first homestand at Petco
-the division clincher in 2005 (personal favorite)
-Trevor's record breaking game
-first two postseason games in Petco
-the game against Seattle where eight homers were hit (Petco record)
-the game(s) last season where we completed the sweep of LA (both at home and at Dodger Stadium)
--the game at Dodger Stadium was also the game where Jake Peavy hit a home run
-the day we beat St Louis in 2005 that ended a eight-game losing streak (it felt like the first win ever)
-Ollie Perez's major league debut
-Ben Johnson's first big league home run
-the game where Phil Nevin pointed up at KT's box
-the game where Phil Nevin complained about Jeff Francis, and what prompted the team to build the additions to the batter's eye

The last few probably aren't too important in terms of franchise history, but they were memorable nonetheless.