I ordered some LEDs a while back from AliExpress - a yellow and a white.
Yellow (5000 mcd) - these are "straw hat" form factor which is the first time I've seen these - I actually like them MUCH better than the standard 5mm LED. These straw-hats sit neatly side-by side in 2-spacing holes on a breadboard, so you can pack them REALLY tightly without issues. I'll be using this form-factor from now on, if I can.
These (yellow) LEDs had an interesting visual - I don't know if you've ever seen a sort of static "sparkle" halo around a laser light, kind of a lot of tiny but stationary dots or lines around the LED output in a halo effect... but these yellow LEDs had that. It was a bit disturbing to look at, but I take it as an indicator that the light is extremely coherent. I'm not sure these are even safe to look at. Anyway, there's that. I'll built a matrix anyway.
White 6000K- 6500K (20000 mcd) - these didnt seem as bright as my earlier "ultra bright" LEDs, but THOSE were advertised at 50000 mcd (advertised, not measured!) but those others WERE extremely bright. But their colour was cold, almost blue-leaning. These new ones are much more pleasant in colouration, so I will also build a matrix with these.
You can't beat these prices - I got 100 of each, for around US$1.75 each, inclusive of postage.