I've been playing Everquest, on and off, for a long time. I think I bought it about a month after it was released. When Kunark was released I didn't get it, then the same with velious... my reasoning for not getting velious was because it was for 30+ and I was only in my teens. Kunark, well, I just thought that there was enough for me to explore without spending more money on an expansion.
I quit and came back a couple times in between now and then, but the last time I restarted EQ I bought all the expansions... and it has changed the way I play. I am now able to participate in any group, as I have all the expansions.
Now, that being said... here is my advice, get them all... if you can. If you can only afford one, get LDoN. It's a great expansion, there is good experience to be had, as well as some good loot, but start saving up for GoD :)