well, all three of your choices so far include an advanced job, so pretty much narrow it down and chose from there.
thf/bst mnk/sam support(lets say brd/whm)
We'll pretend for now that you're just going for the most effective choices, I'm not really sure how mnk/sam works out, but brd/whm is extremely effective, like bard however beastmaster doesn't make a particularly useful sub. Basicly you won't be able to charm anything worth using as a pet, anything bigger will just attack you.
If you want to play thief I'd recomend a ninja as a subjob. Samurai is ok but really thieves are relatively fragile. If you put out a huge weaponskill without trick attack you're going to get stomped. Maybe others have tried both nin and sam at higher lvls and can provide some input.
Monk/sam seems like a decent combo, but as always sam becomes vastly more effective at lvl 30. So using it as a subjob before then may not be nearly as effective as others. Also, while monks have absurd hitpoints they tend to wear paper thin armor. With your hitpoints you'll be able to take quite a few hits, but if you tank too much its detrimental to the party as you end up a mana sink for the healers. Then they heal too much and get agro trying to keep you alive, monster comes after them....bad things happen. Personally at level 50 blm I'm not a huge fan of having a monk in the party. This may change at higher levels, I would much rather have another melee damage dealer (thief/ranger) a tank, or a rdm or bard instead. Its not that the monks don't contribute to the party, it just seems like other jobs do the same thing but better. Again this is from my perspective at my level, your mileage may vary.
Of course as a blm I simply adore bards and rdm's for their debuffing and refresh/mage ballad
Really you need to find a job you'll enjoy playing. The most effective jobs in the world will suck for you if you don't enjoy it. And to deal with the "you can play any job you want, not just the most powerful, as long as you hae fun!" crowd. Of course you can play whatever you want, go ahead and play the brd/drg, just realize you'll be putting yourself, and any party desperate enough to invite you at a disadvantage. There are TONS of very effective combinations out there, more than enough so that everyone should be able to find something they enjoy without putting themselves at a disadvantage