Turtles are generally a very poorly proven group of vertebrates. We have only 5 proven members of this species, all from the Cryptodira. Any Members of the Pleurodira would be excellent candidates for proving. 
Some interesting candidates would be the African helmeted turtle (Pelomedusa subrufa) a species which has been observed engaging in mutuallist cleansing of parasites on warthogs and rhinoceros[i] shown here[ii] . Also interesting would be the mata mata turtle (Chelus fimbriata), with it’s exquisite body shape, used to camouflage itself amoung rocks and streams in South America.
In terms of the cryptodira, a multitude of fascinating species are available. Two separate, but very similar, species of giant Tortoises are extant, the Galapagos giant tortoise (Chelonoidis nigra) and the Aldabra giant tortoise (Aldabrachchelys gigantea). Both can be over 1m long, and upto 250 kg. Geographic isolation on islands. Both of these species are remarkably long lived, with some individual Aldabra giant Tortoises being recorded as living up 184 years.
Other intriguing species for proving would be the pig nosed turtle (Carettochelys insculpta) the only freshwater turtle with flippers instead of legs, and with a remarkle nose resembling that of a pig. Also interesting would be the eastern long necked turtle (Chelodina longicolis) which is remarkable for it’s long, almost snake like neck and it’s ability to secrete a foul smelling substance upto three feet when threatened.
[i] LightMatter. Online Image accessed November 26th 2017. https://en.wikipedia.org/wiki/File:Galapagos_dominance_display.jpg
[i] Owen, J. Turtles groom warthog in never-before-seen behavior. Oct 8 2015. National Geograhic. Online document accessed November 26th 2017. https://news.nationalgeographic.com/2015/10/151008-turtles-warthogs-africa-animals-science/
[ii] Photo by Greg Hume. Accessed November 26th 2017. https://en.wikipedia.org/wiki/File:Helmeted_turtle_25.JPG
Image Source: https://en.wikipedia.org/wiki/File:Helmeted_turtle_25.JPG Photo by Greg Hume.