There is no 'best' class - only personal preference.
Some really general rules of thumb:
1. The newer the bikes, the faster you have to go to be competitive - as you'd expect, really. Personally, my in-built speed limiter (aka the pansy factor) means that I can ride a pre-70 bike just about as fast as I can ride a pre-85 bike... Needless to say, I'm a lot more competitive on the older bikes...
2. The good pre-75 (and older) race bikes are more expensive to buy - but converted trail/enduro bikes are still cheap and still a lot of fun - and they're not at quite as much of a disadvantage as pre-85 trail bike would be against the pre-85 MXers.
The bigger bikes are usually worth more than their smaller siblings.
3a. The older the bikes, the older the riders.
3b. The older the riders, the less likely they are to be lunatics (although it is completely wrong to assume that all young guys are lunatics and no old guys are!).
4. The newer era VMX bikes are better against moderns, which might be useful if you want to to some modern club MX/Enduro work.
But really, its all about what you enjoy riding, and the bikes that you identify with.
Speaking purely personally, my favourite class is/was HEAVEN's combined pre-75 and pre-78 125 class (called "Classic 125"). This is mostly because there's decent fields filled with a stack of good blokes who I trust racing against, and enjoy drinking beers/talking crap with after the racing. Its also A class I can be reasonably competitive in, without pushing myself out of my comfort zone.