Optimal Frank team is a difficult question to answer because there’s a lot of factors that come into play. There’s several templates you can have, different ideas behind it, different situations you’ll find yourself in, etc. I personally favor the Point/Frank/Anchor archtype, and for that you want:
1 a Strong, versatile point character who can reliably get the first hit. Has a simple and effective way to level Frank to 4-5, and ideally an assist that benefits Frank in some way
2 an anchor that provides an assist for Frank’s neutral game (as both a backup in case he gets stuck in level 1, and then to round out his weaknesses in level 4-5), has a backup plan for leveling him, and can cleanup with X factor.
Modifying these templates can be done but come with various strengths and weaknesses. Putting Wolverine on point for example usually means TAC reliance (yes, I know about BLSing off Fatal claw, people still TAC) but while it comes at the risk of them countering it, it usually means the character is dead and frank is leveled if it works; greater risk for potentially greater payoff.
Following this template, I’d say by far and away the best character for point Frank is Dante. strong neutral game, plenty of practical ways to level Frank, and a great assist to back up Frank to boot. Nova’s a strong pick as well, with Nova force being an easy way to get to 4, Centurion M being a great assist, and a strong neutral game (he benefits a lot from shopping cart as well). Skrull’s good too, though stone smite/tenderizer don’t aid Frank that much.
There’s a couple characters that do the job well but I see little reason to pair them with Frank. Zero’s a good example; He doesn’t really rely on high low from a lockdown assist (he just needs something that puts them in blockstun for him to do left/right), so shopping cart doesn’t benefit him much. More importantly, he kills on any touch; what does he need chainsaws backing him up for? Spencer benefits from cart more than Zero, but the same question remains; why do you need Frank when you have Spencer? Vergil is along a similar vein, but more importantly (to me at least), Frank and Vergil are fairly similar characters in their roles; I’ve never seen much need for having both of them on a team. Makes me think of doing 2v2s in WoW with double rogues; if it was a good matchup, it got even better. If it was a bad matchup, you got bodied hard.
Lots of other point characters have options, but they’re lacking in some respect or they don’t have exposure. Spidey’s a good example; plenty of raw tag opportunities, but nobody really pairs the two (that said spidey combos don’t build much hits and hits assists are junk-tier).
The anchor’s trickier. Backup level up plans for Frank, neutral game so he has at least a chance in level 1, and still comeback potential. There’s really nobody who fully fits this bill; Dante’s close but the biggest problem is if you’re using jam session, you have no presence beyond midscreen except zombies (and anybody with a horizontal projectile can hit you out of startup easily). Arthur fits the bill on paper, but…it’s Arthur. Ammy’s good, but again, coldstar doesn’t reach too far. Drones are great but sentinel has no level up back up plan. Vergil’s got the comeback and level up options down, but frank can’t utilize rapid slash that well. I personally use Rocket Raccoon, which has all the neutral game and Frank level up options you could as for, but man does his comeback potential suck. If you’re anything except a 1v1 or a 2v1 with x factor + ample meter, you’re usually in deep shit. Long and short of it, you’re not going to get everything and have to decide where you want to make your sacrifice.
Regarding the specific team of Spencer/Frank/Dante, in some lights I think it’s great whereas in others I find it lackluster. Bee is the greatest example to view for it, and my biggest problem with the way he plays it is his combos. He’s got two powerhouses in spencer and chainsaw frank, yet his damage always seems lackluster. If he was killing and leveling on his first touch, and building two bars and killing with Frank confirms, man would it be scary. I see the potential, but I haven’t seen it put into practice. The other problem is if Spencer dies before getting Frank leveled, playing keepaway against Frank/Dante isn’t hard at all. Gotta respect the THC, and that’s about it. This is offset by Spencer being 1.05 mil health, but still, depending on your team you can either kill him on a touch or I’d argue burn x factor to kill spencer then wear the rest of the team down by attrition. There’s a lot of pieces in that team that can come together for great results (Object toss + spencer alpha assist is a full screen hit confirm for example, love that shit), but I can see ways to dismantle it pretty easily. Still, Dante and Spencer are strong characters, that helps a lot.
Nova/Frank/Sentinel has no backup plan. You’re putting all your hopes into Nova getting first touch. That doesn’t happen and you’re in pretty deep shit. Frank already has plenty of risk/reward going for him, I’d rather try to minimize that than amplify that.
Best team for that template is probably Nova/Frank/Something or Dante/Frank/Something, Just haven’t figured out what the something is yet.