Most PCP's too powerful for standard issue urban backyard usage....especially if shooting up in trees.
And not many guns capable of cleanly killing a squirrel are "quiet". Whatever you get (if you go PCP), depending on how close the neighbors are, will need an LDC to quiet it down. However, there are some 22cal break-barrels that come with integrated sound dampening that won't need an LDC, so I feel like that is a better choice if you on a budget.
Many factors that would determine the "best option". I am a fan of compact carbine style pellet guns, but a rifle will probably be more budget friendly if you have plenty of room for the extra length from your shooting position.
My only advice is try to stay under 20FPE in the city, and even less if your yard/neighbors require it. In Europe they hunt with 12FPE and go for headshots; shortest distance possible if you like be humane.
Many "good" choices but I'll throw out a few.
PCP if on a lower budget, Crosman 3622 w/ added LDC. Buckrail prob cheapest for that, but there are other brands. You can get a slip-on LDC but i'd recommend getting a barrel adapter (Buckrail has those too).
I like the Barra 1100z option for a mid-line PCP (has threaded barrel so should not need adapter for LDC)
And for high-end PCP it hard to go wrong with a Benjamin...you may even consider the P-ROD if you like a carbine.
FOR BREAK BARREL, many choices, but most are Chinese and are hit and miss on quality, unless you want to pay for a Euro-made gun. If buying a "quiet" break barrel that won't break the bank, I like the Benjamin options in 22cal with the barrel-integrated LDC.
Good luck; report back!
