Since it's going to rain all week and trails will be closed thru... What's your best mountain biking experience?

Me, it's riding with my oldest in New Mexico, and climbing up to a prairie at x thousand feet full of wildflowers and butterflies.  He still talks about it, seventeen years later.

R

I rode with my brother up in Maryland for miles open end.   The weather was amazing and the trails were absolutely amazing.   I remember this  profoundly as this was the last time I rode with my brother ...  sadly he passes away a year later..  but the moment still lives on...   

Other than that riding outside of Vegas was pretty crazy

small sample size, but what i read, is that it's not as much about the ride, but the experience with the people: relatives, friends, other (aliens), which i 2nd!
and then, there's the beer afterwards.

2000 - Backcountry mountain bike tour in Utah with one of my best friends and 10 strangers for a week. Best part of the trip was hiking Angel's Landing in Zion National Park and camping under The Watchman. Lots of laughs and lots of beer!

Many for me:

Pisgah, Boone, Kerr Scott.
Coldwater Mountain. Santos, 
Terlingua, TX: Palo Duro Canyon, TX. Taos, NM, Gallup, NM
South Mountain in Phoenix. Flagstaff, Sedona.
WHite Rim in Utah.
N Rim Grand Canyon.
Copper Canyon in Mexico.

Would be hard pressed to name a number 1.

! got my first mountain bike in 1983, so my sample size is pretty large. Here's a few that stand out:

Combining the Deer Park and Allamuchy Boy Scout Camp trails in NW Jersey into an all day epic that included a connector through a giant drainage pipe under Interstate 80.

A snow ride on Christmas Eve in 88 when my brother asked me to be his best man

The Mt Snow Norba National in 1994. 102 degrees at the start of the race, and a mid race jump into the snow making pond made it memorable. I also got to creep on Susie DeMattei

The first Olympic race in Atlanta. Not a ride, but what a day.

The 2nd edition of the 24 Hrs of Canaan where we won the Sport Class and finished 9th overall. The highlight of my racing "career"

A week long trip to Gunnison and Crested Butte with 2 buddies in a Saab with a bad O2 sensor. The trails, most notably The Luge at Hartman Rocks, made up for the Saab.

A shop trip to Tsali for some camping, trails, and camaraderie. 

Reuniting with a high school buddy to hit some iconic SoCal trails while on a business trip.

-Last day of Poplar Tent, laps with so many people until it was too dark to see the trail any more

-Lap at Beatty with my then 3 year old on his 12" Hotrock. We come up on a group of walkers at a pretty good rate of speed, and he jams on his coaster brake for a stylish skid to a stop. His smug face, and the reaction he got.

-Renting a 5010 from Santa Cruz HQ, riding from the cliff overlooking the beach up into the hills around UC Santa Cruz for about 4 hours, and hooking up with locals for some truly wild bootleg trails, Mailbox was the one that stood out. (You need to Login or Register to view media files and links)

-Every Pisgah ride

These comments should illustrate to anyone that hasn't been to "the mountains" yet that they have to go!  Nothing gets you stoked on the sport quite like it.  It's like skiing in Colorado instead of Boone, surfing in Hawaii instead of Wilmington, etc...

For me it was my first trip to Fruita, Lake Tahoe, Moab, Interbike in Vegas.   We started at Lake Tahoe and did a great road trip south.
My first Pisgah ride left a big impression on me too (it was the loop that probably everyone does the first time) Bent Creek.   
My first trip to Wilson's Creek was a blast.
At Poplar Tent I worked up my courage for a good month of riding before I hit that ladder drop.  Then I was shocked at how tame it was   
I stayed in the Waterfall Cabin at Reeb Ranch for a long weekend and I highly recommend it.   
Winning my first Mtn Bike race was a good feeling.   

Now I am looking forward to my kids getting hooked on the sport so I can create another decade of memories!

Best ride?  All of them.  Even the ones I hated as they teach me so much.
Or anytime up in George Washington NP.

On the Kenai Peninsula in AK, coming back down the trail fast from Lost Lake with my oldest - heard him (behind me) shout "Whoaa!"; stopped the bike and turned and saw a bear running straight down the trail at me - freaked - realized it was Baby Bear, quickly said a silent "Whew!', but then freaked worse. Baby Bear went off the trail up slope, mewling away, and sure enough, Mama Bear was on the downslope side of the trail - mad as hell  - snorting and bumping trees. So me and my cub had got between Mama and her cub. After about 5-10 minutes of Mama heading back up the side of the trail towards my cub (and me actually telling myself "If Son gets mauled by a bear, Wife's gonna kill me."), she finally crossed the trail. Wife 'bout killed me anyway!