Have you considered the Sentinel feat? If you're trying to be "tanky" and keep nasty monsters off your fellow PCs, Sentinel is incredibly useful.
If you're looking specifically to boost a stat, then Con is probably the best choice. That said, if you don't find yourself running out of hit points very often, you might consider boosting Dex so you can be more effective at range. Javelins are nice, but you run out of them fast and their range is super limited. Sometimes you really need a longbow. +1 to your initiative doesn't hurt either.
Incidentally, is this a build that you are planning, or is this an actual PC who has (or is about to) hit 19th level?