To get started, here are some popular Christmas tree reticles:
Horus
H59
Horus
Tremor3
Leupold
FFP CCH
Nightforce
MIL-XT
Primary Arms
ACSS HUD DMR 308
Vortex
EBR-7C
When shopping around, here are a few things to consider.
Mark spacing: Most (mil-based) tree reticles have branches at 1 mil intervals. The H59 and MIL-XT reticles have fine dots interspersed at 0.5 mil intervals halfway between the branches. You might like this or not, but the extra reference points do help a bit when you're holding over.
Illumination: Some illuminated reticles (MIL-XT, EBR-7C) illuminate much or all of the tree. Some (Tremor3, H59) only illuminate a few points on the tree.
Hold-under: You may prefer to have tick marks on the vertical crosshair near the center dot, for holding under for closer shots, or for ranging targets. Horus reticles typically lack tick marks immediately above the center, but most other tree reticles have them.
Low magnification: All FFP reticles have to strike a balance between fine precision at high magnification, and being able to see the reticle at all at low magnification. Really fine reticles are nearly invisible at 3x, while those which you can easily see at low power can obscure the target, bullet trace, splash, etc. at maximum magnification. The Leupold CCH reticle addresses this balance by making three of the crosshair legs into thick wedges. If your scope doesn't go below about 5x, then this is less of a concern.
Ballistics: A few Christmas tree reticles have some ballistics data incorporated into the tree. The Tremor3 has dots corresponding to wind drift (independent of caliber). The ACSS HUD DMR has wind dots and ballistic drop, but only for .308. A somewhat extreme example, the
Tubb DTR has dots corresponding to wind plus the vertical interaction between wind drift and trajectory, but only really works for a few 308 loads, and is only available in a couple of (rather nice) Leupold scopes. If you handload, you may not want to have extra ballistics data in your reticle, but wind dots can be calibrated pretty closely to any load in any caliber.