I have OM-1, OM-3, OM-4 and OM-10, I had OM-2 and tried the Om-4Ti.
As already said the one digit models are the good ones and I think they were all very good when they come out. Now, a few decades later some of them might have old electronic's problems.
Briefly. The OM-1 is all mechanical and is the only model to have mirror lock-up, the meter however is not very precise and they use discontinued batteries.
The Om-2 has aperture priority and the same "usual" meter but they use LR44 batteries.
The Om-3 is mechanical but has a highly sofisticated meter with spot, highlight and shadow mesurement. They use rather old electronics and many of them dry the batteries in a few weeks, mine included. Even keeping the shutter speed to B till the last minute doesn't help much so for me it is battery in just before using and battery out once it goes back into the bag. Apparently The Ti version (very rare and expensive) has sorted this out but cannot comment since I never had one for any extended period of time.
The OM-4 has the same meter as the OM-3 but sports and electronic shutter. Apparently it also suffer from batteries problems but mine (old beaten up and bought second hand) doesn't which leads me to think that maybe there are also OM-3 without this problem out there. The Ti version should be better. Since for all of them when electronic if gone is probably very hard to repair (but you can find the main board and many spare at high price on specialized sites)
I would go for a mechanical model: OM-1 or OM-3 and since of the two the one with the best meter is the OM-3 I would go for it, also since the Ti version is better I think that if your question is really "which is the best" I would say the OM-3Ti, but that's expensive.
On the other hand any other model should produce beautiful pictures, so if you find one at the right price go for it.
The OM-3 and Om-4 have also a usual hotshoe if you use it, OM-1 and Om-2 had a funny accessory shoes (OM was supposed to be a completely modular system) which breaks easily and is har to find.
The OM-10 which I have works perfectly but it is only aperture priority unless you buy another strange accessory (or you build it yourself if you are into those things). Mine resisted a fall from some 50 feets during a mountain trip (but falled on grass) with no problems, so the build is not bad...
GLF