I’m interested in what everyone’s favourite matchboxes are. I haven’t had time to try them all out, but will definitely check out anybody’s recommends.
Tell us the name, briefly what it does, and why you love it.
I’ll start:
LS_airglow - Essentially the most natural looking glow available. So easy to control and with curves to control the fall off. Does everything you want a glow to do. haven’t used anything else since I found it.
One of my favourites is crok_renoise. I use it pretty much all the time.
y_lensblur (fast dof / camera blur)
LS_Airglow (exponential glow)
y_blurs (blur inside alpha only sampling those pixels)
JB_multimatte_manager (manage puzzle mattes in one node)
cook_renoise
MasterGrade
Ls_Vops to combine many alpha
Ls_Dolface Amazing Blur able to restore edge
Y_dirblur the bi-directional Blur is so usefull
Infill Blur to quickly fill area with color around
Ls_Ash nice adaptative sharp tool
Ls_Posmatte to better manage alpha with 3D
MatteViewer. Not a creative tool but for diagnostics in a complex comp it can be an absolute treat.
Frontset Offset in Action, most used matchbox, but I still know some artists prefering the slow oldschool way.
LS_Lumps (Frequency Seperation)
LS_Wireless (mostly for hair beauty and removal)
crok_beauty (even if aebeauty outdated it, sometimes it has better results)
Y_Sharpen,
LS_Airglow 
LS_Vops,
crok_vignette,
Y_lensBlur,
crok_grunge,
K_Chroma
the_noise
and MasterGrade
(EDIT) I was just following the crowd but “in the spirit” I have highlighted my FAVORITE 
y_source. Not the coolest matchbox. But in my user bin since years and years and one of the most used. To perform front offsets in beauty jobs without adversely affecting performance. Unlike action’s source nodes.
In the spirit of the question I’m only picking one:
y_blurs. Between the matte blur and the grad blur I use it for so many cool things. a truly powerful node.
Y_lensBlur,
TheNoise
Ls_Airglow
K_BlurMask
crok_grunge
crok_renoise
crok_distort
additivekeyer
Crok IBK
If by “favorite” you mean gratuitously overused…
Airglow
In a situation where you would use pixel spread to push out the edges of the front of a green screen element, try replacing the pixel spread with an inverted Infill Blur Matchbox to blow out the edges. You’ll be surprised.
Spline blur is an amazing tool too. Especially for hair work.
one that nobody mentioned yet,
Ls_UVewa,
you can control the amount under and over + filtering of any distortion with an uvmap.