Hi Niner24-7. You'll see many tailgaters in all of the lots, but you'll definitely have the most fun if you park in the main lot. The most hardcore fans are there and everytime I go there you can usually count on meeting some friendly 49ers fans to hang out with. In addition I've found it quicker to get out after the game when parked there. If you want to get into the main lot, you'll have to either have a prepaid parking pass (that season ticket holders have the option of buying before the season), or get there pretty early- before 10 and you should be OK.
I'll be at the Seattle game with a buddy of mine and we'll probably be there early to grill up some food and drink some beers (much cheaper than the $8 beers inside the park!).