Obeche is probably the most used wood in mouldings. There are also woods like Tulip that are very similar. It can vary quite a lot - some is very soft, almost like Balsa. Botanically speaking, it is a Hardwood. As is Balsa.
In the past, Ramin was a favouite wood. Close-grained and consistent colour and takes little effort to put a nice polish on. In recent years there have been import restrictions though and now it can only be exported ready milled. Not in log or plank form. Some suppliers still stock Ramin moulding, but only in a very limited range of profiles - unfinished.
Oak and Ash. Ash is probably easier to work with than oak. Some oak can be as hard as hells door-knockers.
These generally require a bit more effort to get a decent join. Oak/Ash mouldings are mostly too small to get a biscuit slot into, but there are machines (and someone will tell me the proper name) that cut dovetail slots in the mitre face which allows you to bang in a sort of bow tie-shaped plastic thing. The machines aren't cheap and you would have to be a regular Oak/Ash user to justify buying one. Some companies will do a chop service with the slots cut in already. Bit iffy buying Oak/Ash on chop though. You often need to have the option to match up grain patterns.
Clamping the join and using the time-honoured hammer and nails is not a bad idea.
The other popular wood is Pine. Which covers a large range of woods. There are lots of different species that come under that general heading. Some is very coarse-grained - depends on how fast it grows. Some is well seasoned some is just kiln-dried and can shrink or twist. Some is very knotty while some has no knots at all. All these are classified as Softwoods, although they are generally harder than most Hardwoods. They don't have open pores like Hardwoods and can suffer from splits and cracks.
Rough knotty pine is quite a nice finish in itself, if you want the rustic look. Sometimes it's used with a thick gesso coating to cover up the defects and variations. A common practice is to laminate strips like a Battenburg cake and join short bits with a finger joint. Finger-jointing on a single length can result in weak spots. I've seen some with ten joins in a ten-foot length. But combining finger joins with laminating actually makes a very stable and strong moulding. Pine has a tendency to twist and bend more than Hardwoods.
Other woods sometime crop up. Maple, Cherry and such. Beech is quite nice. Harder than Obeche, but more softer and more evenly grained than Oak/Ash. Woodworm love it though.
