Olympus OM and in particular the OM1 and OM4Ti - since 1978.
Today I have both cameras still and use them as often (read every day) as my Pentax LX, and recently the ME Super. It is a dangerous question as one cannot escape to compare cameras, within one brand already, and for sure between brands. For me the perfect camera does not exist.
One large plus for OM in general is the placing of the shutter speeds ring around the lens mount. It means one can operate the shutter speeds in the exact same way as one operates the aperture ring. And that means you do not take your eye away from the finder anymore.
The needle metering of the OM1 is so simple and effective, it is the reason why I will always find the leds the lesser solution.
The viewfinder of the OM1 is what I find most special. It is so large and bright. I installed a Beattie screen and had the lightmeter adjusted for that. With the Zuiko 40/2 this is a lot brighter than my LX with 50/1.4. Not all brighter finders focus more easy, but this OM1/Beattie combination is really something.
The Olympus OM1 has the reputation of setting the trend for the smaller slr in the 70ties. And the quieter one. Perhaps they started it, the OM1 is from 1972, but it is the Pentax ME (1976) that is the smaller one, in size and weight. And it is more quiet. Personally, I am not looking for the smallest camera, I find the OM cameras exactly right. Let's say they are the same size as the Pentax LX, a little less wide and a little lighter perhaps.
The OM4Ti is another story. Almost the same metering range as the LX, also with off-the-film metering. But with a beautiful combination of spot metering and ae lock. I am not going to explain here how it works, but it is totally easy and, once used to it, intuitive. For an automatic mode camera (one can btw also use it in manual) I find this one coming close to being perfect.
Except it has turned out to be a fragile camera for me. The body I am using now is the last remaining of 4 bodies, that all got electronical problems in the past 7 years. Perhaps it is because I am not very careful with cameras. Anyway, it is the reason why I got the Pentax LX and why after that the different Pentax cameras became alive for me.
The lenses are a different story. I find all Zuiko and all Pentax lenses fine. For me the focal length is more critical and over the years the 40mm has become the lens I simply leave on the cameras. The Zuiko 40/2 is not as small as the Pentax 40/2.8 and it is why I prefer the Zuiko.
So, like Nesster here, OM and yes also Pen FT, are impossible to imagine to be without. Having said all this, the LX is great and so is the ME Super . . .
PS: between OM4 and OM4Ti I had the Leica R4s for almost two years, also quite nice. Except people always fuzzed about the brand being so great, etc . . . I quickly went back to nice anonymous OM