How much money do you have? Best would probably be 2k+. FWB is a good start, but there are a few that are arguably no worse. Pretty much all the top line stuff is pcp, so you will need the equipment for that as well, so add in $400-$3k (depending on how throughout you want to do it).
There is a smaller version for only about $1200
Cheap side, best is a Russian SSP. It is $500ish, but no extra equipment. It is almost as good as the top end stuff, just a bit less adjustable. IZH. (Ops, now closer to $600)
Once opon a time, the least expensive pcp was the AA, It was about $600
There is also the Crosman 1701P, nice, but not as good as the IZH, plus you will be investing in pcp support gear.
Then comes the Avanti stuff - it is really week, but shoots acruate enough. It is under $200.
I don't know if Crosman makes a CO2 with the 1701P trigger. I think it is almost a bolt on switch - that might be your cheapest bet. If you do it, understand that you will be doing a breach change ($40), a sight change ($40), and the trigger change ($60). So it adds up quickly.
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I just got a HB22. If it was made correctly, I would recommend it (or an HB17-both are essentially discontinued). Nice wood, decent trigger, it is actually pretty accurate - plus it is brass so will last a lifetime. Unfortunately either Crosman is activly trying to kill it by selling seconds intentionally, or QC is so low that everything is a second. Damn front pin was drilled off square. That in turn made the front sight cant. I am going to have to hang the rear sight almost 1/8" off center to compensate, and make sure I have it twisted as I pump to keep it from binding on the side of the pump slot.
Still It is managing about 1" offhand (2hand) groups at 10m, and for me that is about as good as it gets. Plus, as a pumper it makes a great little hunting pistol.
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Just configured a 2300kt at the custom shop. $200 and it still has a questionable trigger group. The one I configured was a 2300T with walnut grips ($160 msrp - with plastic grips, plus retailers might discount it since it is a stock product, not custom shop - ok NOT).
Put a 1701p frame and trigger on it and it would be a nice little pistol for under $250.
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As an afterthought, here is one other.
IIRC it was originally a German pistol, but now is being sold as a $40 Chinese clone with the Beeman name still attached (Chinese version is a P17 - I think the German version was a P3). When it works, the reviews are pretty good.
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I have a modified 2240, modified 1377, HB22, 1701P, AA Alpha Proj, and an FWB 65 (that is long since discontinued - but a work of art. You might be able to find one in good used condition for $600ish - these were 1980's vintage). I have shot the Avanti, I have been arround the IZH, but not shot one. I have no personal experience with the Beeman.