Her best bets are the following, depending on how you want to play.
for the slow and passive heal, and its items are inexpensive in the long run. Only if you want to play more defensively due to its low stat increase, but it can be used offensively with the right items. Not exactly the best to rush if you opponents are experienced and know Trist's playstyle enough, since it doesn't really to much for pushing or defending by itself.
for a dedicated everything-bully playstyle. Its recipe's items are expensive for early-game, but it pays off with its passive (not to mention that at full charges it increases pretty much everything Trist wants).
for a dedicated minion-bully. It's easier to use than Guinsoo's Rageblade, and offers increased Atk. Speed from the beginning. It's also one of the items you want for a Crit Bully item choice.
if you're annoyed with minion blockage. It doesn't do anything Runaan's Hurricane doesn't do, except for that and its Lament passive.
is probably the best rush fully-built item if you want a Hybrid or AP Trist. It's been a great item for me, especially when you use her Q, and it's mostly-easy to build. Not to mention it builds up from
, which is always a great item for Trist.
For lesser items, you should try going with items that increase your Atk. Speed (
/
) or AD (
/
or
). If you plan on building based on your lesser items, it might be a good idea to rush
or
.
After any of these, you can build anything you want to capitalize on. I'd personally just start with Nashor's Tooth or Guinsoo's Rageblade for the best gameplay...unless you end up feeding early, then you should go Runnan's Hurricane to get CS.