There are four exits at Mo Chit station... Which one of the four is best to exit to catch a taxi to Don Mueang?

I'd use exit 1, taxis are at the bottom of the steps.

The other way to do this is use exit 3 and cross the footbridge. A couple of taxis will be waiting at the bottom of the steps across a rope barrier - but you have to cross the bus lane to get to them. The advantage is no queue.

The other way to do this is use exit 3 and cross the footbridge. A couple of taxis will be waiting at the bottom of the steps across a rope barrier - but you have to cross the bus lane to get to them. The advantage is no queue.

This sounds like cheating? ;-)

...

getting taxis there has been a nitemare the times I tried. could never get one

This is most likely after the event but definitely always use exit 3 especially if it is after 12pm when the escalator is reversed to the down direction for the remainder of the day. There are parked taxis queued at the bottom of the escalator and waiting for pax. You are also north of the bus stop near exit 1 and will thus not be impeded by buses. I'd suggest that 95% of people needing a taxi to head north to go home use Exit 3.

"The other way to do this is use exit 3 and cross the footbridge. A couple of taxis will be waiting at the bottom of the steps across a rope barrier - but you have to cross the bus lane to get to them."

This is usually true but there is no need to cross the bus lane and the rope barrier given the queued, parked taxis at the bottom of the escalator. However, it is an option for those without luggage.

Exit 1 has the distinct disadvantage of being blocked by parked vans and you are behind buses which are stopping at the bus stop and can accordingly end up being blocked in should a taxi actually be able to collect you there.