Narrow experience does not imply that one should dismiss everything so quickly! You pulled one pretty awesome god and one decent utility earlier and wrote them off as useless before you even figured out what you could do with 'em, and you immediately dismissed Siegfried and Cu Chulainn as 'worse Valkyries' before even considering their uses-- and thought negatively of all of the above. I get that you haven't done anything other than Valk since you started, but you're going to need more than one team to handle stuff (trust me), and everything you've got so far will be a fantastic step in that direction.
Basically what I'm saying is be a little more positive about the stuff the game gives you, because in most cases, you'll be able to use something somehow.
Siegfried is the key leader of a very solid team setup. Used properly, he's amazing. He also makes a really good sub. In fact, a whole lot of physical monsters do, because they're great HP stat sticks when you have open space. Of course, you'll more often prefer stuff that has subclasses too to hopefully make up for the lack of RCV (some phys monsters even have NEGATIVE RCV, like Gigas-- but he makes up for that by having one of the highest raw HP stats in the game).
Cu Chulainn is great too. The vast majority of balanced type monsters in the game are wood element, so you'll have a lot of synergy to work with. And really, replace pretty much everything in the previously mentioned link above with their balanced counterparts and you'll have a solid team there too. You'll have less raw HP to work with than you would in a physical team, but you've got a huge utility variety to work with and will have much more workable RCV too.
Basically, all the heartbreakers (Valk, Sieg, Cu Chu, Gigas, Vampire) are solid members of any team in some way or another. Valk/Sieg/Cu Chu make good leaders as well as subs, and Gigas and Vampire are better subs than they are leaders (if not only because B/D Vamp is ridiculously weird because his leader skill boosts dark types but his main element is water and that's kinda awkward (but not unworkable, mind)). Normal Ult Vamp as a sub is good enough as he is. Or use him as a leader if you really want for a 0/4/4 team. But that won't have the raw power of the other leaders.
Keep in mind that in most of these cases my lack of interest in this stuff isn't because the monster is terrible, it's because it's not useful to me right now. I'm at a point in the game where I need a second kind of team to handle different situations, and I keep getting stuff that, to me, looks like a way better sub than leader. "Amazing sub to some mega-rare god you don't have" doesn't exactly thrill me right now. To me it looks like I have a surplus of subs and nothing to do with them, which is kind of frustrating. But if that's not the case, great.
If I can manage to get a Berserk before Two Heroes goes away I think I basically have a solid Physical team. I'm short a great green physical monster, but I can use an ultivolved ogre for now and get what seems like a very Sakuya-friendly team full of orb changers and all of the right colors.
Cu Chu is a little less clear right now, since a lot of the Balanced monsters I picked up randomly seem to have weak synergy with him. Maybe I'm just missing something right now, but isn't a bunch of orb changers on a mixed team kind of a waste? It seems like orb changers would be good for either teams that are heavily built around one color or combo-making teams.
Beast Rider seems like a total waste of space no matter how I look at him right now. I don't have any particular way to make Attackers a thing and I can't ultimate evolve him any time in the foreseeable future.
Apollo seems like he fits well onto a Siegfried team if I make one, especially if I go with Dragon Emperor Siegfried and try to go for the Sakuya-friendly build that I sort of have the pieces for. He doesn't provide any colors I don't have, but there should be a fair amount of light on the team between Siegfried, Gigas, Sakuya, and himself, and I don't otherwise have a light orb changer. And he gets rid of the dark orbs I don't use.
Vampire doesn't seem amazing for much right now since his synergy with Cu Chu seems weird and my teams don't seem like they'll have a lot of dark on them. I have considered taking a shot at the Batman machine to try to get Batarangs and make a 4/4/0 team like I was hoping to with one of the Greek gods, but Batman's lack of an ultimate evolution is unfortunate and I don't want to risk getting a bunch of Catwomans and Robins. I'm sure he'll go somewhere, but I don't know where yet.
