There are kind of a lot of variables to consider: what level you're starting/ending at, and whether you're allowing multiclassing, feats, and variant rules (including Unearthed Arcana and splatbooks.) That makes it hard to give much of a definitive response.
Without multiclassing, I'm probably most drawn to valor bard or tempest cleric, both of which seem to play pretty nicely on their own. I'm not sure if straight paladin counts as a gish here -- it doesn't get full caster progression, but has a decent number of magical tricks, so YMMV -- but if so, that's another very strong option.
With multiclassing, I'm a huge fan of (Ftr 1 or 2) or (Paladin 2) with Warlock x. Starting from level 1 and playing up, I think Ftr 1/Warlock 5 is the better way to go, then picking up Ftr 2 at any point after that. (Ftr 1/War 4 is pretty painful when the other players are all getting their level 5 class abilities.) If you're starting at lvl 10+, Pal 2/War x may be the better way to go, as the Second Wind isn't as useful and you potentially have more, higher level slots available for smiting.
With feats, Heavy Armor Master is fantastic for warlocks, who get access to lots of easily renewable temporary hps and end up making very solid tanks.
I haven't looked much at splatbooks and UA options, because we don't really use them in our games -- we basically use core rules with feats and multiclassing. I have the impression there are some neat tricks you could pull off, though.
Without multiclassing, I'm probably most drawn to valor bard or tempest cleric, both of which seem to play pretty nicely on their own. I'm not sure if straight paladin counts as a gish here -- it doesn't get full caster progression, but has a decent number of magical tricks, so YMMV -- but if so, that's another very strong option.
With multiclassing, I'm a huge fan of (Ftr 1 or 2) or (Paladin 2) with Warlock x. Starting from level 1 and playing up, I think Ftr 1/Warlock 5 is the better way to go, then picking up Ftr 2 at any point after that. (Ftr 1/War 4 is pretty painful when the other players are all getting their level 5 class abilities.) If you're starting at lvl 10+, Pal 2/War x may be the better way to go, as the Second Wind isn't as useful and you potentially have more, higher level slots available for smiting.
With feats, Heavy Armor Master is fantastic for warlocks, who get access to lots of easily renewable temporary hps and end up making very solid tanks.
I haven't looked much at splatbooks and UA options, because we don't really use them in our games -- we basically use core rules with feats and multiclassing. I have the impression there are some neat tricks you could pull off, though.