That's a big debate and it all depends on your wants and conditions.
CO2 is great because it's a liquid and at normal room temperatures it's like 700psi if I remember right so until you run out of liquid it's going to be fairly consistent, downside is under about 50F the pressure drops pretty good, some people will go as far as getting motorcycle handlebar heaters to help...
Spring piston (be it gas strut or coil spring) are great because of the great power in the simple mechanism but the "magnum" springers are more hold sensitive and the cheaper ones have single stage triggers so you have to have a very consistent light hold while pulling a 12 pound trigger... that's where the higher end like the RWS come in and the CDT triggers for many of the others the less strong shooters are easier to shoot.
pump guns are kinda my favorite but I rarely shoot them anymore but I don't shoot much anymore, the nice thing is most are pretty simple (mainly Crosman, some like the Daisy but I find their mechanisms more delicate and odd, Benjamin and Sheridan are tried and true solid workhorses which no one will argue about). They're nice because if you're just plinking you can pump like 3 times but if you are hunting you can go bonkers (some people go crazy like 100 pumps in some builds, 10-15 is a good maximum for most) for some power. The downside is it's a lot of pumping and there's less options than there used to be...
PCP is where it's at really if you have they money for the options you want... most are adjustable in many ways and have shrouded barrels and regulators and magazines and whatnot, prices range from around $200 to like $4k... you can tune them for power and get very few shots per fill (I think the Dragon Claw is rated at like 9 shots a fill but is 50 cal...) or tame some down in .177 and get like 100 shots per fill like I'm pretty sure I could do on my Air Venturi Avenger that I have toned down to as slow as it will go with the stock adjustments...
The high pressure option is SCBA tanks or whatever the purpose built ones are called, they're rated at like 4500psi...
One of the reasons I like Crosman is parts availability and most of their guns are considered "lego guns" like the 1377 or 1322 pump pistols and the 2240 CO2 pistols (and it's various factory builds like the 2300T and whatnot), for example the builds in the next couple pictures... the one with the scope is a 2240 with a I think 24" .177 barrel, stock valve, steel breech and the Crosman offered butt stock (I have the part number to get it through parts at like half price...). The second pic is a Crosman 2289 (same as a 1322 but different pump handle and a quick release butt stock) with a longer barrel, then another 2240 with a longer than stock barrel... that 2240 with the long .177 barrel gets like 60 shots per CO2 cart and is quiet, I've taken many starlings with it...