If you already have a good pedestrian vehicle: a good bike lock.
I (and lockpicking lawyer) recommend the kryptonite evolution. Good balance of price and protection.
For a step up I recommend the kryptonite fahgettaboutit.
Either way, bolt cutters aren't getting in, and portable angle grinders will take a little while because the chain is annoying to grind.
I see too many expensive bikes chained up with thin little locks.
This is the one I'm talking about, $75
Kryptonite Evolution 1055 Mini 10mm Chain Bicycle Lock , Black https://a.co/d/0xjPXNw
If your bike is worth over $300 then imo it's worth it, now that every douchebag has a mini bolt cutter in their backpack (around here)