Crucible seeing as you are above 450. It basically hands out B grades like its candy, even if you aren't good at pvp

First thing that I have to ask, what's the goal you are looking to accomplish with running a Solo Op and getting a B or greater score? Is it only for the daily / weekly portal challenges that require a B score? Then I would say that Quickplay Solo Ops is always an option since it is locked at at 300 PL and set of modifiers. The runs tends to require little to no effort to complete. Just make sure that that are doing the optional objectives for that run. Now the drawback, most of the rewards drop at 400, but they are T4 / T5 depending on your overall PL. If it's more of the need for the T4/T5 at or above your current PL, but not worried about the bonus drops, then select Caldera on Ultimate. You'll need to play around with the modifiers to find the ones that you are most comfortable to deal with in your runs. I for one will always run the No Starting ammo and Hot Step and then select the Faction Unit and Faction Drop Don't overlook the Banes, but again select the one that you are comfortable dealing with on your runs. I avoid any that are have the mini-Screebs (hate the buggers) and the Shielded Banes. Give you an example - Caldera I only have to select Hot Step; Vex: Shocker; Air Superiority and No Starting Ammo and that gives me a B with enough wiggle room to throw on a Boon or two. There is also the Fireteam Quickplay, but at least for me has always been a hit or miss on getting a B score. The Arena Quickplay is also a good place to get a B score.

Once you get to 550 one bane mod will give b grade mod or one bane and a tier 4 will give b+. This is on ultimate difficulty. It doesn't need anymore as thats all you need to get 550 drops at tier 5. If you are below just keep playing with the modifiers to get to b+.

You can do Caldera on Ultimate. Use no starting ammo as there are crates every ware to refill your ammo and add deadly solar threat with adding three solar mods on your chest piece and you can get through it easy. I use Prismatic warlock with Hellion and feed the void with the Getaway artist. exotic Using a weapon with with heal clip also helps. Finish the objective, I avoid the Precision Kills one, take out 4 overload champions and you should finish with at lease 3 min to spare. This should you at lease an B+ . If you use no modifiers you should get at lease a B

Arena Vanguard Alert Quickplay

Hate to be the barer of bad news but you have to put on loads of daft modifiers that make it too hard to be worth bothering with , I did a solo ultimate exotic quest earlier today, i only got rank c from the chest and thats from dieing twice to bungies phalanx shields pushing me of the map ,so even doing ultimate isn't worth it unless your using modifiers which make it even worse

Solo ops, do the highest difficulty you have unlocked and put on enough modifiers to get B rank.