Asking me to pick my favorite F13 is like asking a parent to pick a favorite child... Assuming they're one of the parents from Cheaper By the Dozen. That's just cruel.

Parts 1 through profile pic. Others are ok, but the original 5 are all I need.

1 and 2 are the knock out classics!

I count Part V as much of the F13 franchise as Halloween III for the Halloween franchise....no Vorhees.

If I were to rank them:
 

The first and fourth movie were always my favorite. If I were to rank them it would look like this:

4 (The best in my opinion)
1
6
2
7
3
FVJ
Reboot
8
9
5 (I count this movie as much as Halloween 3 was about Michael Myers)
Jason X (The worst in my opinion)

I'm actually midway through rewatching the series (I'm watching part 7 tonight) so I will post something definitive when I have finished but part two is currently my favourite.

Part two is my favorite too.

That will change. Part 7 will be your new favorite.