Latest stable versions, in plural, as two supported and updated branches, is libtorrent 1.2.5 and 1.1.14. The 1.2.x branch features more improvements, but has incompatible internal API changes needing code-changes to software using it, compared to the older 1.1.x branch which has had a longer development/testing/use cycle so possible can be more stable in some scenarios as been tested longer, I believe, hence two stable versions provided, and 1.1.x is still updated with fixes albeit at a slower rate and main development effort is 1.2.x branch and just keeping 1.1.x around for compatibility, e.g older software not updated to the new API changes, or if just finding 1.1.x more stable for whatever reason/circumstance/usecase.
For latest windows deluge, v1.3.15, only 1.1.x branch supported, so 1.1.14 is latest supported for that. Deluge2 on the platforms available i.e pretty much only Linux I believe, though there is unofficial macos app curtesy of Lord-Kamina, and manual install instructions by Cas for windows, has been updated to support both libtorrent branches, and some distros use 1.1.x while others choose 1.2.x, often depending about how "bleeding-edge", or new, the distro is.
There's a newer than deluge's provided libtorrent, curtesy of Nekotan here, also including built openssl, though he stopped updating it some months ago because supported openssl was outdated from then on, and so would be missing any later published security fixes, but atleast newer than officially supplied libtorrent, and actually quite possibly latest supported libtorrent already, which seems likely considering he edited his post with download links, last time the same day latest supported libtorrent released i.e 1.1.14: viewtopic.php?t=55168