I haven't played him very much till now, but I'm a huge fan of relics and I'm not that much of a fan of ancient coin. But I have to say as Rakan you're able to choose between all 3 support items, depending on your playstyle and on the enemy.

Relics is good if the lane is not easy and you need the additional heal. Additionally, Relics has much more Utility (imO) because you have far more health and the shield.
Using the Stacks isn't easy because you don't have the execute but in the early it's ok (i have ad in my marks for that) and in mid/late you either don't need it anymore or you can use you q for it. But you need either a good/friendly adc or a premade adc so that he let's you take the last hits.

Ancient coin is good because you don't have the problem with the stacks and can focus on poking and the map etc. But i think, that you don't have that much of an impact with it and while the active of the coin is nice for the ult, it's in my honest oppinion a terrible, terrible idea to use Rakan's Ult as engage on it's own because he will never get the same resictances as any other tank, a) because he's support and b) because he's ranged. I thin Rakans Ult is better for disengaging, flanking and as followup after another engage. BUT if you have something like Rakan/Ori/Malph, Rakan should - matter of factly - engage.

Spellthiefs is good if you can anticipate or know that you have a winning lane and need the extra bit of damage and know that you can poke them to hell and back. It's problematic in teamfights because you are neither a damage dealer like Zyra, nor are you a tank, so you are mostly suiciding when you w/r into the enemy.