Mopani wood, so far as I have experienced, doesn't rot. It's rock hard and dense, which typically means it will last a long long time under water. Mine is still leaching some tannins after nearly 3 years in a tank.
Malaysian driftwood is also rock hard and dense. Both of these release tons of tannins at first and keep it up to some degree for awhile. If you don't want that, you can soak them first or boil them. I've soaked wood for as long as 6 months, and even used bleach to speed up the leaching of tannins. All depends on what you are trying to achieve.
Any wood that's been allowed to dry out is less likely to sink right away and soaking will waterlog it so it will sink. If it won't stay down, tie rocks to it, that will speed up the waterlogging process.
One thing I did learn is that grapevine, which is sold for terrariums and reptiles, and is also rock hard and heavy, doesn't sink well and once you finally get it to sink, it rots quite quickly. The dark heartwood will rot out much faster than the lighter coloured outer wood does. So I wouldn't bother with grapevine for a tank again. It does not release nearly so much tannin as the darker woods from Asia do.
Almost any aged hardwood can be used for a tank though, and if it's been soaking in a bog for some time, it will sink, if nothing else. How much tannin it leaches may depend on how long it was wet and where it was. Peat bogs have a lot of tannins and the wood may still leach a lot in a tank.
If you are really wanting lots of tannins, leaf litter is a good way to get them. Indian Almond leaves are especially rich in tannins and release them quite quickly too. They rot fast as well, so they need replacing fairly often. One leaf will turn a few gallons of water to steeped tea brown in a day or two. You can use oak or other hardwood leaves instead, so long as they are gathered after they turn fully brown, though they have less tannin than the Almond leaves do. They also last much longer.
