The 920 is their flagship model (until the 925 - coming soon to Msia - and the 1020 arrives). I've the 920 and absolutely loves it. It's also their most expensive model now (around rm1599 I think).

The 620 was released in the same time as the 920 and meant to be an affordable model for those who do not want to splash for the 920. But mind you, the 620 is better than a lot of other non-WP phones (which are only famous by namesake).

The 520 came later and is Nokia's answer for a low budget model. If your dad is using smartphone for the first time, suggest you get him the 520. It's cheap and yet it has the same experience as the other Lumia (620/920) except maybe you don't get all the fancy bells & whistles like wireless charging on the 920 and the 520's screen is not as bright as the 620. But to a first time smartphone user, he won't be looking out for these bells.

I think Maxis is still running the promo for 2-yr contract with a free 520.