Triclopyr is one of the best choices for woody species, but also way more expensive than glyphosate. I mix triclopyr and diesel fuel, 2/3 to 1/3 gives you the spot spray strength recommended on the label. Basal bark application (spraying just the lower foot of bark) works on trees smaller than 4", also works better on smooth bark than rough bark trees. Hack and squirt works even better, but it's also a lot more work. Before I'd do hack and squirt I'll just cut the trees off totally with a chainsaw, then spray the stump with triclopyr, I can do this faster than hack and squirt because I'll cut a large are then spray the stumps all at one time. On bigger trees I'd sooner drill a 1" hole on a downward angle and do like Okie, squirt straight glyphosate into the hole, and the tree will die. Last spring I did a 53 acre invasive spraying project with 60 to 70 gallons of spray mixture, including a lot of basal bark spraying, and the results were close to 100% with the above mix.
Amines vs esters: a simple laymans explanation is that an ester version is a more volatile and more potent delivery method of the same active ingredient, and usually costs more. A few important things to remember between the two; the waxy surface of a lot of different foliage leaves are penetrated better by the ester version of the herbicide for better kill rates. If you are tank mixing with another herbicide always use the ester, the amine doesn't mix well. Wind drift is much worse with the ester, especially in hot weather, so when spraying in hot weather use the amine. If spraying basal bark in the winter I'd be ok with either one. Remember, the active ingredient is the same % in the two, it's just in a different formulation.