Forgotten Gods is better than Ashes of Malmouth in the sense that it adds the Shattered Realm, but it requires AoM anyway. I wouldn't say Ashes is a *good* expansion, it fixes basically none of the problems of the base game, it simply adds more of the same quality content. Well, you do get access to +experience potions to level up alts faster. The thing is, you don't play GD for the content, which isn't very good I'm sorry to say due to the aforementioned map and story issues, you play it for the variety of builds in a grim dark world that somehow always has a happy ending but not really, along with challenge content like Crucible, Roguelike dungeons, super bosses, Shattered Realm, etc. Forgotten Gods is the better expansion because it adds the SRs on top of the content that is the same quality, but at least has an Egyptian theme this time around and you aren't swirling around tight 19th century concrete domiciles. Don't get me wrong, I like me some 19th century vibe, but the map just isn't goooooood, ugh it makes me a bit angry thinking about it, lol.
Anyway, what is good about the expansions is that they add variety to the usual fields, forests, swamps and same-y dungeons of the base game. Try the base game and you'll see if you like it enough to buy the expansions or Crucible. I'd advise getting Crucible either way because it's a good arena mode that lets you test your builds while getting juicy rewards doing it.
I'm making it sound worse than it is, GD is definitely a must-play game even if you have a cursory interest in this genre (or perhaps video games in general).