Modern defense players - what line do you most fear to see?

As the title suggests, I'd like to hear from experienced modern defense players what line they find the most unpleasant to play against. It could be objectively challenging or challenging from a human standpoint, maybe you just don't see it often so you don't feel confident against it, however you want to interpret that...

Or, to put it another way - if you had to play white against a modern player what line would you choose and why?

I prefer not to book up players of the White pieces!

I would say lines with f3 like the 150 attack or early h4, but that might just be my burden to bear

Not currently playing the Modern, but I've played the Modern some off and on. To me it's when White plays the Austrian Attack. Actually, I choose to dodge the Austrian Attack by switching to the Gurgenidze (a variation of the Caro Kann) if White is placing his pieces so that the Austrian is possible.

In other words, after 1 e4 g6 2 d4 Bg7 3 Nc3 if black plays 3...d6 White can play 4 f4, the dangerous Austrian Attack, so here I go 3...c6 instead, preparing 4...d5. If White moves 4 f4 anyway then 4...d5 punches a hole in White's dangerous pawn formation.

If White has played Nf3 early blocking his f pawn then ...d6 is on instead of ...c6.

The catch is the Gurgenidze can lack in counterplay, but to me that is better than getting steamrolled.

A subtlety I like here is the waiting move 4. h3, asking Black to clarify his intentions. If ...d6, showing an intention to contest the dark squares, I just go ahead with f4, and the h3 inclusion is useful to prevent Bg4. But if ...d5, I play e5 and the position is better without having played f4, as this would weaken the light squares and limit the scope of the c1 bishop. Of course if Black plays f6, I can play f4 at that point.

OP, if you want a one-size-fits-all solution against the Pirc and Modern, you probably want the Austrian. I like to play a 150 Attack type setup against the main-line Pirc and Tiger's Modern (1. e4 g6 2. d4 Bg7 3. Nc3 d6 4. Be3 a6) so I use move-orders which allow this, which I think is better but can get a bit complicated. Basically the inclusion of the move ...c6 is my cue to switch to an Austrian setup, because this move is useful for quick Q-side expansion with b5, a5 and so on in the event of opposite-side castling, while in the event of a straightforward battle over the center it doesn't do anything and hampers Black's natural development as now Nc6 is prevented and Bb7 is bad. In the Pirc specifically, I also switch to f4 ideas if Black plays an early Nbd7, as this demonstrates an intention to contest the dark squares with e5.

Basically the idea is to wait for Black to commit to a plan and then set up accordingly. The oddity here is Tiger's Modern, because a6 is a useful move for Q-side expansion, yet I don't play an Austrian. The reason is that (unlike against ...c6) Black can switch to ...b5 and ...Bb7 and the big center is a little vulnerable to the firepower of the bishops. Unlike in the Pirc, Black has not committed to Nf6, which takes the sting out of any e5 advance, which typically is the plan at some point in the Austrian.

Actually there are a few Averbakh setups which are pretty playable- especially the one involving...exd4 and ...Ne7/Nbc6.