above first, if you've got 20k to spend, then you can talk to 'real' vault people. They sell and install the same stuff that goes into your local bank. Its available, but not cheap.
1st tier - graffunder or brown
2nd - any TL-15 rated safe
3rd 1/4" or thicker plate safe
below this level by on price, because you're really only keeping out the smash and grab guy with a screwdriver and a hammer.
also get a monitored alarm system and insurance that covers the FULL REPLACEMENT value of whats in the safe.
_ANY_ safe is only buying time.
These days with cordless saws, drills and grinders, anything less than 1/4 plate or special composites (like amsec BF) can be breached VERY quickly by the most novice of thieves.
REALLY consider anywhere but the garage for a location. Even if you bolt it down, 2 guys a stolen pickup and a chain, and that safe is out of your garage in < 60 seconds.