Lots of different factors to consider with this question.
It’s very hard to cover classic 5*s now. So much dilution in the tokens and only a 5% bonus chance. It’s conceivable to get a few covers over a few months with this method, but it’s pretty hard to get more than that.
If you really want to cover 5s, then the best way is probably to save up 2k in cp and and a hundred+ LTs and then going crazy on a good set of 3. Unless you earn a few hundred LT pulls every 6 weeks that is just about the only way to really champ a 5 from scratch now.
As for overall utility. There are a few considerations too. Some 5s excel at transition play with 4s, others are good with some 5* partners.
For all-around play i would offer this list (based on a mix of my own experience with undercovered 5s, fighting different 5s in the game, and discussing the game with alliance mates on Line).
(1) Thanos (eats healthpacks like nothing else, but current fastest)
(2) BB (with 5 black he gets to pretty substantial passive damage very fast)
–small gap (next group is all sort bunched up as usable and very solid with the right partners)–
(3) hawkeye (not enough instand damage to be a #1 tank, but a great secondary damage character. Almost impossible to deny)
(4) BP (plays up with thanos, a bit hard go use otherwise)
(5) BW (hits very hard, could use a battery)
(6) strange (great stun, owns goons, very squishy)
(7) GG (good all around, especially with the new CD-based characters)
(8) im46 (excellent 4* teammates, pretty self reliant)
(9) phoenix (5* cascades are still good, and so are cheap strike tiles)
(10) cap (so much better after the fix, but still not great)
(11) BSS (not bax per se, but less useful in the current 5* meta which doesn not care about strike tiles)
(12) SS (immune to stun is awesome, itherwise outclassed by newer characrers)
(13) OML (still great way to save health on trivial nodes. But noticebly underpowered against real opponents)
–gap (this is the current 5* trash tier)–
(14) star lord (not enough offense or self accel)
(15) banner (fun to play, but noticeably worse than the rest of 5* land)
(Note: doc oc will likely fit into the botton of the list. Question is proabably: “is he lower middle class, or trash tier?” his blue takes too much time/ap/board luck to he really good, and his green is way too complex for its own good. Black seems like a slight worse version of strange’s stun)
And i second everything jarvind says below about scaling. Champing 1 5* will take your maximum level pve opponents from about level 330 to about level 375. Adding more 5* champs pushes you up over level 400 opponents.