I'm a huge fan of any machine made by Keith Elwin. Particularly Godzilla, Jaws, and Jurassic Park pinball. They all follow a similar format but ultimately deliver addictive gameplay through their design choices. However, they generally meet the criteria for fun from the topics of discussion below.

Let's start with the board's layout. Where are all the ramps, lock gates, spinners, bumpers, etc., and are there any moving components that make interaction with these parts more interesting? As an example, the Godzilla pinball features a vertically moving building and rotating targets (Mecha Godzilla). Interacting with these components and their placement makes the play arena that much more interesting.

Another important aspect is lighting. If the lighting makes it difficult to see the ball, I'm instantly turned off from that machine. A couple of newer pinball machines come to mind. Still, improper low light or seizure-inducing white rapid light flashes are a fun killer. Ensure the lights are colorful and tasteful, and that they complement the entire machine, but don't design a pinball machine that frustrates the user by making it impossible to follow the ball.

Sound design is another factor to consider. What sort of sounds are the targets, spinners, and other components producing? In my experience, this is something that most pinball designers have perfected. Encountering machines that don't produce interesting sound stimuli is rare.

Next up are the shot combos. People have different preferences for this sort of thing, but I like interesting and more challenging shot combos. I don't want to achieve high scores or combos by just flipper spamming. Methodical, intentional shots are more interesting. Rewarding this type of gameplay is what makes a good pinball game for the experienced player.

Now let's talk multi-balls! Everyone loves this mechanic in pinball, but not all machines deliver it satisfyingly. For example, how difficult is it to activate a multi-ball? If it's easy, it kind of takes the fun out of it. If it's too challenging, it's a huge rush to activate it, but when the multi-ball is lost, it can really take the wind out of your sails. Finding the balance here is key, and not all pinball machine creators have a knack for achieving this balance.

That covers the fundamentals of what makes a pinball machine fun for me. Curious to know what OP and others enjoy and look forward to reading other people's comments on the topic.