If you were only to own one spring pistol, the old Diana 5/5G is a good candidate. Better than almost anything you can buy now, except an LP8 or HW45 (which means £260+ new).

Fairly powerful, not too heavy, well balanced, decent trigger, good build quality (everything bar the seals lasts forever), decent accuracy. Arguably a bit boring, but in a good way.

The old (off-white) piston seals turn to cheese over time . Time-consuming, but not hard, to strip, clean out the bits of Brie and Stilton, and change them. The new piston seals (red or blue) do not turn to cheese.