Any particular TYPE of sim?
This is a bit like asking for a recommendation on A Game Where You Come Into Conflict With One Or More Enemies (within which you find RTSes, FPSes, Beat Em Ups, SHMUPS...)
MS Flight Sim is generally considered the dogs dangles as far as flying a plane around goes, but as far as I can tell it's mostly been gradual graphical tweaks for the last 20 years.
LFS as mentioned is a pretty good racer (good to see it's still going ... never got round to buying the full version and figured I'd missed the boat).
If trucks and the like are more your thing, and you have a reasonably hardcore PC, you could check out Rigs of Rods. ANYTHING with integrated graphics will likely die a horrible, protracted death, bleeding from the gut, mind, and you'll need at least full XGA to read some of the on-dash keybind cheat sheets. Don't trust the start screen if you get 60fps ... add a couple vehicles and go to a tricky bit of terrain and that could well drop. I've been down to 1fps on my laptop; really, below 10fps it all gets utterly pointless, particularly as you can't really join in with multiplayer games properly.
Gran Turismo likes to sell itself as a sim, but the strangest thing is how it pays a shocking amount of attention to some things (the car meshes, things spraypainted on the tarmac at the nurburgring, certain engine samples, suspension settings that even F1 engineers probably tweak according to trial and error, 83 squillion different subtle variations of a dull toyota sedan that was only sold in Japan, and only between 1996 and 1998) but is equally stunningly deficient in others (license test difficulty weighting, operation of rev limiters / manual clutches / LSDs, total lack of "real" automatics, rubbish lighting shaders, using the same engine noise for probably 25 different cars of different cylinder capacities and cylinder numbers or arrangement (a 2-cyl boxer should sound nothing like a V6), speedos/tachos that are massively out of line with the speed/rpm that the car you're driving can actually achieve, near-constant tyre squeals (you know HOW HARD you have to corner/launch/brake to make that happen IRL, even when racing?), next-to-useless dyno graphs, "feel" of the analogue controls vs how the car controls react, inability to get a decent-looking body-coloured wing for a car that actually HAD ONE IN REAL LIFE if you'd ticked the right option box, egregious and gaping holes in the model inventory for anywhere that isn't Japan (I have yet to find a satisfactory - nevermind "exact" - match for any car I've ever DRIVEN, let alone OWNED), stock gear ratios that seem to have come from a guy tripping on acid on the motherfucking MOON... oh yeah and the fact that damage was only finally implemented in GT5 and you STILL can't turn the car over or run very far off the fascistically designated track). I paid for GT4 and 3 quite happily (4 as an "upgraded 3", pretty much - that one was REALLY minging), after losing weeks of my life in 1 and 2... But I can't be bothered any more. Haven't bought 5 (would require a new console anyway), probably never will. May give 6 a look if it ever arrives, but it'll need to fix the leftover stupidities.
So yeah, I wouldn't believe the GT hype. It's an enjoyable enough racing game but it's not so good of a sim.
Other than that .... urrrhm. I'm not really au fait with modern simmy things. I could talk your ear off about things released approximately '85-'95...