If you enjoyed Valkyria Chonicles, the second and third ones on PSP are also pretty good. While the second one is in English, the third one is Japanese only. Although there is a fan translation patch/ISO available online somewhere. I think the third is actually my favourite so far for it's "Dirty Dozen" premise alone.
The two PSP God of War games are fantastic, probably my favourite of the series. Although I played the HD versions and not the originals, so not sure how they hold up there.
Metal Gear Portable Ops is great fun if you are a fan of the series. It plays much more like the older PS2 titles, with a very early version of the eventual Peace Walker/MGS5 soldier collection mechanic. Story is decent, and I believe somewhat canon to the series.
I would also recommend Peace Walker, but that has had a HD re-release so you may have played it elsewhere. If not, definitely get it as it is a sublime portable title. It's pretty much Metal Gear 5 before 5 was a thing, with a full canon story with Hideo at the helm.