The three easiest nations to do a WC with are
- and Mughals,
in that order.
Austria into HRE
Austria starts as Emperor of the HRE with a force limits slightly above 40 and has missions and events to get PUs over Hungary and Bohemia, as well as a gold mine (in reality a silver mine, but the game only knows gold) in Tyrol.
By guaranteeing every prince, as well as warning every country bordering Sundgau you can ensure peace within the HRE. This will get you massively over the limit for diplomatic relations, but diplomatic power (mana) can not fall below -999. This means for a while you will be unable to take land without claims, but can still conquer territory and add it to the HRE. keep in mind that waging war for more then 10 month per 1 IA is not worth it, because you get +0.1 IA/month for being at peace.
With boni from getting re-elected (put your king in the army and drill to kill him faster), you should be able to get the second-to-last reform by 1515-1525, which will get you the vassal swarm. Put your focus to dip points, you will have caught up in dip tech by 1550 - 1560.
At this point, you are absolutely unstoppable. Your vassal swarm can crush everything. you can be at constant 100% OE, but all the while also feed your vassals. Once you are able to form client states, do so at the border of HRE territory and feed the whole world into vassals. Once you have the whole world under the control of your vassals, click the last reform to become the HRE.
This is the most easy, most boring, most secure and most braindead way to do a WC and is basically just a big grind without many areas of luck or skill involved.
You can also use the timeline exploit to get 2 more provinces right at the start and to ensure the game starts with religious peace, disabling League Wars and reformation so you do not need to deal with that at all.
Castille into England into Spain
If you own at least one of Dharma or Wealth of Nations, plus have both Golden Century and Rule Brittania, Castille is the powerhouse of the game, due to their missions and events.
- [Event] If you disinherit Enrique at game start, you ensure a very quick Iberian Wedding, granting you PUs over both Aragon and Naples (if Aragon still holds Naples, which they should at that point)
- [Event] You have a good chance to get the Burgundian Inheritance, giving you access to the Lowlands
- [Event] You have a chance to get an Habsburg Heir
- [Mission] You get a mission to PU Portugal (do so after they have taken Exploration and are on the Historical Ideas track, ensuring they also take Expansion)
- [Mission] Once forming Spain, you get a mission to PU Austria, who often also hold Hungary (via their event)
- [Mission] By invading England and forming England, you get access to the english mission to Subjugate a 200% Scotland and PU France
By 1550, you can realistically have formed Spain and hold the thrones of Naples, France, Austria & Hungary, as well have Norway (via the 1st Age ability to transfer subject for half the price) and Scotland as strong vassals - effectively meaning you can colonize uncontested and at the same time eat Europe if you so desire. Alternatively, you hold enough land to become Emperor and feed provinces into the HRE to pass reforms and quickly gain the vassal swarm as well. But at this point you are strong enough to just brute force the WC, if you do not wish to deal with the HRE.
Having Portugal under you with Exploration & Expansion means they colonize for you. If you also colonize and collect in the English Channel, you are very well off financially and can field large Mercenary armies as well as +5 advisors, making you very well off in terms of both manpower and mana.
This requires a little more skill then my #1 pick of Austria, since you need to defeat England, for which you need to be able to land troops on the English Island, and you need to defeat France twice (once to get the claim on their throne, once to establish the PU) and you need to manage AE in the HRE when PUing France. You also need to dismantle Kebab by brute force, which needs to be started quick enough if you wish to avoid truce-breaking.
Timurid into Mughals
Much has been written about Mughals. In the late patches, they have become extremely strong. A Timurid -> Mughals run is more challenging then both above, but still a very easy WC if you know the basic mechanics of the game. Their -25% Core-Creation-Cost (CCR) paired with +5% discipline and -10% Idea Cost as well as -5% technology Cost places them in a very favorable position mana-wise. Their mechanic to assimilate cultures of which they hold all cores plus the -1 Unrest from their Ideas make them very stable, and the boni from assimilating some cultures are also superbly strong (another -10% CCR from hindustani, +15% Institution spread from germanic, 10% morale from japanese and more, see here). The Mughal missions offer many claims to start you off as well.
This requires more skill then #1 and #2, because you actually need to play the game and wage war/conquer, but with their extremely strong Ideas and other Boni, Mughals is hands-down the strongest nation in the game. Austria is only better because of the very broken HRE mechanics, and Castille is only better because of their insane missions if combined with english missions - they aren't stronger on their own.
The most challenging part of playing Mughal forming them when starting out as Timurids, as they start in a quite unstable position, as well as planning and executing the move of the capital to Europe to enable trading companies in Asia and to reduce stress through corruption by having too many territories. Although this can also be dealt with simply by rooting out corruption. Especially the start is why I didn't put Mughals at #1.
Of course there are way more nations which are very capable of doing a WC, but those three are by far the most easy. Prior to the latest patch (1.29), a Manchu -> Golden Horde -> Mongol Empire WC as also a good contender, but I am not certain how the lastest changes to Manchu (especially to banners) might have changed this.
Ottoman starts in a very strong position as well, and doing a WC with Kebab is certainly on the easier side of the scale. A somewhat disagree with becoming Catholic. By ensuring a draw in the Religious Wars, you can become Emperor as Orthodox nation or Coptic nation (Orthoman or Coptoman), or you can use the Sunni mechanic of spreading the religion through trade nodes if you stay Sunni. All are good choices with different strengths and weaknesses.
England is also a strong nation, although not as strong as the others. By PUing France you start off quite strong, and you can keep the HRE catholic by crushing the reformation centers afterwards. But honestly, starting as Castille, forming England and then forming Spain is just plain better.
A note on Poland: As Christian nation (Austria or Castille), make sure to have your dynasty on the throne of Poland via the Elective Monarchy when they abolish it and and form the Commonwealth. They'll be a monarchy, have your dynasty and no heir when they do so, offering the opportunity to quickly claim their throne and wage war against them. For both the Austria and Castille run, this is a nice bonus making a WC even easier.