If you don't own a Convoy yet, I suggest you start with a S2+. The S2+ was accidentally my first ever "custom" flashlight, and they are very good quality for the price, and cheap enough to buy several to test them out. :)
To describe my own purchase deciding process, I am first asking myself some questions - most importantly, what will the light be primarily used for?
After that, it's details like the beam pattern and colour temperature / CCT (Kelvin).
For indoor use, I'd recommend a floody pattern - I got the S21D's daddy, the 3X21E, 519A both in 1800K and 4000K, and both with a 60° TIR (full flood).
And with 3x 21700 cells, the 3X21E works both as a portable work lamp and emergency lantern with excellent runtime. But it's big enough to not the be the first light you grab.
In the M-series, I am very much considering this - M21H, SFT-40 3000K
MAO Convoy M21H , 21700 flashlight
convoylight.com
I am told it has excellent CRI, and I am in general preferring warmer lights (3000-4000K). But to me, this will be an outdoor walking light, adjacent to my Emisar K9.3, E21A 3500K + W1 Green.
As far as I can tell, the entire M-series has deep reflectors, all of which indicates a throwy beam pattern.
For something jack-of-all-trades, I would say something like the S21E, 519A 4000K, 30° bead TIR.
Convoy S21E green 21700 flashlight
convoylight.com