Extension is the most popular of the talents to be picked, and it's not hard to see why. It is easily the best talent pick for most games as it provides a passive buff which doesn't impact core gameplay too much. Choosing Extension is never a bad pick, especially if you plan on being the main healer on your team.

Restraint fills a very niche role in very few games, but you should always be careful when deciding to pick it. Base this almost solely off the composition of the enemy team. If there are 3 or more supports and a tank, then this is likely a good pick. If you believe the enemy tank could out-heal your team, then you may want to go for it. But overall, this perk is unimpactful at best, given that Envelop lasts 3 seconds with a cooldown of 14 seconds. Even if it was perfectly timed, healing will still be available for the enemy almost 85% of the time, while you are forced to stay in range as well as reduce the healing effectiveness of your team. Building cauterize will usually be a better option.

Focus is a very special talent of Rei that I personally use almost every match but I would not recommend. It is by far the best perk for 1v1 situations and can be used if you decide flank Rei is a wonderful idea.
On first glance it may seem this talent is advisable as the overall effects of Spirit Link are greater than normal. For ultimate boost the standard Spirit Link will net 3.5% ultimate charge every 7 seconds whereas Focused Spirit link will provide 4.5%. The statistics are misleading however, as this ignores 2 major factors that are essential to Rei's loadout.
Firstly, it does not triple the percent boost that linked players receive from Chain Heal. This alone would not make it bad but you would have to perfectly time each chain heal in order to get the boost at all, considering that allies would not be linked for over half the match.
Secondly, switching targets quickly is very helpful for Rei for both Envelop and Vivify, as both are powerful when linked to the right ally. This also factors in the fact that sometimes Spirit Link will not link to the person you aim at, so being able to switch to the intended target is ideal.
This all makes Focus the worst talent, right?

Well, not exactly. In theory Focus is the most powerful talent with a coordinated team, but it is extremely difficult to use. It is, however, great when deciding to flank or 1v1 flanks. Linking to an enemy can provide great boosts that can help you take down all but the most powerful tanks in the game. With the right cards, Spirit link can deal 200 damage per second, slow 30%, and straight up cancel 20% of damage done to you by that player. Provided you link to an enemy first, this beam has infinite range, almost guaranteeing 600 damage on an opponent (some opponents such as Atlas can unlink through time travel). A 30% slow will almost completely nullify champions such as Maeve, Victor, or Evie, making Rei the queen of killing flanks as a support.
Focus comes at the great cost of being a terrible healer for your team, but provided you spam Spirit Link enough, it may be worth it.

Aggressive Rei actually works very well to counter any other squishy champion in the game. In particular, annoying characters such as Maeve and Victor are hurt hard by Rei's set.

My 4 go-to items normally consist of Chronos, Moral Boost, Veteran, and Haven; a combo which prioritizes keeping me, and therefore my team, alive. Often Veteran can be substituted for other items of preference such as Resilience if the enemy has a lot of CC or Nimble if you are feeling able to avoid more attacks and want to keep up with the team. After cauterize was removed, the built-in anti-heal makes Rei's ult much more valuable than before. Moral Boost helps keep Rei relevant as a healer throughout the entire match.

I exclude items such as Illuminate due to Rei's unique ability to track invisible champions such as Skye or Saati through Spirit Link.