If buying new, the 6-36 is the latest and greatest and is supposed to be excellent, even if the FOV is artificially limited in the US market for patent compliance (but then again, the FOV on the ZCO 5-27 and Tangent 7-35 are also limited to just under 22 degrees apparent FOV for the same reason.) The big issue with the 6-36 is you better like either the the gr2id or P5FL reticles, because right now those are the only 2 reticles available. My recommendation would be the DTII+ turrets.
The 3-27 is an older scope but is a good option if you intend to use it with a clip on thermal or NV because of the 3x bottom end magnification, but optically it's not as good as the 6-36.
No way would I consider buying a new 5-25 at current prices when the 6-36 is in the same price range (some 6-36 configs are cheaper than certain configs of the 5-25) and the 6-36 is a better optic.
Buying used, a 5-25 at around $2250 +/- is a good deal, but at that price your selection will probably be limited to the older DT turrets and possibly older reticle choices. Still very good scopes mechanically and optically, but not everyone likes the older reticles and some complain about the tunneling from 5-7.5x. The DT turrets are also a bit fine on the hash marks and can be hard to read depending on your eyes.