Victoria 2 for me as well (great to see it popping up in the thread

), and I'd bet good money on HoI4 early next year, but if I had to choose between EU4 and CK2 it'd be EU4 all the time. Biggest reason for that is that EU4's gameplay evolves a bit over time - it's different playing a nation in 1780 than it is in 1480 - wheras beyond a few changes to the various modifiers in one direction or the other, CK2 is CK2 whether it's 769 or 1430, so a long game of CK2 feels very one-note. That, and a family in CK2 is a family. There are a few different rules to spice it up (Muslims and decadence, for example, or starting early on with gavelkind succession) that help make it a bit more interesting, but the things you do each game seem very similar each game regardless who you play as. It wasn't much different for me playing my Byzantine to Roman empire game as it was playing Lanka to conquering all of India. Whereas my Novgorod to Russia game in EU4 felt different to my Poland game felt different to my Portugal game.
That, and there's too much time spent looking at, sorting, and scanning lists in CK2. Better functioning (or even better, just less) lists, and CK2 would have stood a better chance.
They're both great games though, would recommend playing both for at least 20-odd hours to see if someone liked them

.
More depth, no dependence on mana, no artifical limits on player expansion...
I'd actually argue less mechanical depth (although far longer lists!) in CK2, but that's a bit subjective, but all games, everywhere have a dependence on mana. EU4's 'mistake' is that the 'mana' is too obviously mana, so people are struggling to understand what it means. EU3 still had mana (one point every ten years per slider), CK2 absolutely, definitely has mana (prestige accrual, piety accrual, money accrual). Mana is just a 'stock' of some kind of power of influence in a game. In EU4, the 'mana' is an abstract for the Government's capacity to act in three different fields, and the model is actually pretty elegant, but it's also a bit abstract, so not as easy to understand, so people go 'it's unrealistic and mana', when actually it's more realistic, in many ways, than the previous approaches to power and influence accrual in previous EU games.
I'm not saying its perfect, and it's clearly a bridge too far in terms of requiring that leap of understanding by many of EU4's players, but that doesn't make it a bad model, it just makes it a hard-to-understand one. Don't get me wrong, I'm not saying it's perfect (there's a fairly arbitrary and clunky break between financial power and capacity to act in admin, diplo and military fields, when money could in fact be used to enhance capacity here, and more than just by spending it on advisors - over the period, many states used greater finances to develop institutions to assist in this endeavour), but it's hardly 'it's all unrealistic, because it's mana'.
As for limits on player expansion, how about a vassal limit, demesne limit, fixed numbers of holdings and fixed limits to how those holdings can be developed? These are no more or less artificial than AE and overextension (note - I'm not particularly a fan of how overextension works in EU, but that's because they have to fudge it because of no mechanic to the internal power and admin structure).