Diplomatic is a good early group to get. It gives you an extra diplomat, an extra relations slot, and the ability to break off RMs without the stab hit. These are all brilliant bonuses for acquiring PUs. It also gives some other really nice things like dip rep which you will need later on to inherit/integrate, and improve relations which just all around excellent. Subsequently, you will want Influence and Admin. Influence gives more dip rep and crucially 25% discount on integration costs. Admin gives you one of the best ideas in the entire game which is Adaptability - 25% reduced coring cost (and coring time, a frequently undervalued aspect) and more importantly for PUs it has a 20% integration discount policy with Influence which totals to a 45% discount. The faster coring also means you suffer the effects of OE for less time and one of the effects of OE is a malus to dip rep. If possible Plutocratic ideas will also give you a policy for a further 10% discount but you won't have access to those unless you started as a Republic. If you can get it, then you end up at 18% integration cost with full admin efficiency which is really quite good if you PUed a coloniser and need to integrate them.
One thing I will say, though, people often agonise about the integration process but it's often better to keep a big PU as a subject. Ask yourself, can you actually make enough states to really benefit from integrating them? Does the subject have some strong military ideas that you are benefiting from? Is it likely that if you wait a bit a play it right that you could inherit them for free? Once you get to the absurdly high levels of dip rep attainable in the mid-late game it becomes possible to inherit even a fairly large subject, and when you inherit the provinces don't get slapped with 60% local autonomy.
Others to think about are Humanist which is just generally an amazing idea group for its ability to prevent revolts but also grants two extra promoted cultures which will come in pretty handy post-integration/inheritance if you want to get the most out of the land. You'll also want to consider some of the stronger military groups if you're planning to force PU a strong nation. I really like offensive for this - it gives a nice balance of quality and quantity and remains relevant and powerful throughout the entire game. Offensive combined with a strong set of NIs (like France who is one of the best nations for a PU game) should give you all you need get it done militarily. That 20% siege ability is seriously, seriously good. Especially now that the AI is so fond of counter-sieging and wars can sometimes come down to a siege race. Combine with Innovative once you can squeeze that in for an OP policy (1 siege pip, 10%siege ability) and the very useful (and somewhat hard to come by) -0.05 monthly WE.
Which reminds me - I sometimes like to take Defender of the Faith in order to get defensive CTAs for nations that I want to protect because I plan to PU them in the future. It allows you to protect them without incurring a pesky truce like an alliance or guarantee will cause. It also grants a set of bonuses that are more than worth the cost.
The TLDR answer is: Diplomatic, Administrative, Influence (probably in that order).
