Front Mission - Build a squad of wanzers (mechs) and destroy enemy squads of wanzers while surviving global government power struggles. I've finished FM1 on DS, FM3 on PS1, and FM4 on PS2. All were excellent and a fabulous time. As far as strategy goes, these games are never too hard or too easy, falling squarely in the middle. You always have to try to win, but never will you feel like you have no hope.
Front Mission on DS was my favorite of the three I've beaten.
Valkyria Chronicles II - Help a group of young military school cadets fend off an invading army while dealing with racism issues. I beat this on PSP, and loved it. People tell me it's not as good as the first game, and that must mean the first game is unbelievably good, because this game was a blast. (I plan to play the first and third game within the next two years.) This was another SRPG where the difficulty was always right in the middle. I enjoyed the gameplay so much I beat every single optional mission (there's a lot of those).
Final Fantasy Tactics - I'm not even going to try to encapsulate the story in one sentence here, it's too complex. I didn't play this game until a year or so ago, and I expected to be disappointed after seeing it hyped up for so long. Turns out my fears were unfounded, and I LOVED this game. Granted, I played the PSP translation injected into the PS1 game (best localization / no slow down). This game is crazy crazy fun and is addicting as hell. It's not too difficult, but there's a massive difficulty spike in the middle of the game... and then strangely the difficulty regresses from there back to being easy again. Fantastic experience not to be missed.
Fallout Tactics - This one is terribly underrated. I think people have problems with it because it will kick your ass and not care. Yes, this game is quite hard, but in a fair way. What people don't know is this game has a lot of RPG to go with that SRPG tag. There are sidequests within missions, hidden missions, your player characters are highly configurable and level up in complex ways. There's moral choices to make, all that good stuff. I loved this one years ago, and I was absolutely addicted to it to the point of finishing its 65 hour campaign in one week. If you love
Fallout, and you love tactical strategy, you will love this game. People used to complain about the computer AI turns taking too long, but with modern processors I doubt that's an issue anymore.
Faselei! - A war story where you play as soldiers in an elite mech division. Yes this is another mech SRPG, although it's not much SRPG---more tactical turn based squad really. It does not play like
Front Mission though. Man this game is tremendously fun. You set up your own mechs in your squad by preprogramming them before each move. Because you have to preprogram your moves in advance for quite a few steps, you must do an excellent job of planning ahead. As in, you don't just plan one move ahead, you need to plan three or even five moves ahead. It's easy to learn, and not too hard to master. This game can be challenging, but once you get some nice chips for your programming setup (and rocket fist punches) it's not too hard. Such a hidden gem this game is though! Definitely my favorite Neo Geo Pocket title.