It really varies from Assault to Assault. My best advice would be to take a group of 5 or 6 people with good job variety, since they could win most any sort of Assault. Do a variety of new Assaults to rank up, as the higher rank you are, the more options you have. Sometimes (often) you have to get good and high in the ranks before you get to a nice easy spammable Assault.
Also note that bringing more than 3 people isn't quite the hit to points that it used to be when Assault first came out.
Then:
3 people: 100% points
4 people: 75% points
5 people: 60% points
6 people: 50% points
Now:
3 people: 100% points
4 people: 90% points
5 people: 80% points
6 people: 70% points
As for which are more spammable, and which are trioable, and which are trioable *and* spammable, that's a matter of personal opinion, and my general observation is that Assaults get easier or more spammable the higher rank you are. (PSC Assaults, on the whole, suck. The sooner you can rank up from there, the better.) Some opinions of the ones I've done:
Leujaoam Sanctum:
PSC - Leujaoam Cleansing - reliably winnable with 6, probably a little tough with 3. Kinda time consuming.
PFC - Orichalcum Survey - not reliably winnable in the first place.
SP - Escort Professor Clavauert - quasi-reliably winnable, but a little dicey with aggro management, esp. since prof. heartclot doesn't stop for you. Kinda obnoxious to spam.
LC - Shanarha Grass Conservation - reliably winnable with 6 and probably even with a few less if you're careful about pulls. Note that points received are based on how much grass the rabbits manage to eat before you kill them all (so 3 people would kill more slowly and probably lose potential points as a result).
C - Counting Sheep - reliably winnable with 3 unless you get *really* unlucky, but it's somewhat more luck-based to get max points by getting all the sheep unless you have more people (still luck-based, but pretty good odds then).
Have yet to try S/SM/CS.
Mamool Ja Training Grounds:
PSC - Imperial Agent Rescue - *very* luck-based *and* obnoxious. Recommend doing this one as little as humanly possible.
PFC - Preemptive Strike - a mini-cannon (healer and 2 good melees) can trio this, bit safer (and easier to get in under the time limit) with more people. Fairly spammable as a result.
SP - Sagelord Elimination - this one's more annoying than difficult but is pretty easy to win reliably even with a small group.
LC - Breaking Morale - easy enough to *win*, but for max points you have to find lots of treasures. How spammable this is depends on how good you are at MGS'ing and not letting mobs see you.
C - Double Agent - ridiculously easy and spammable, you've got half an hour to track down the right qiqirn, just be sure not to accidentally capture the wrong one before you're sure.
S - Imperial Treasure Retrieval - this one's quite a bit of fun (it's basically capture the flag), but it's not *quick* and the points aren't very reliable.
Have yet to try SM/CS, though SM is reputedly a lot of fun for melees.
Lebros Cavern:
PSC - Excavation Duty - spammable with 6 but you'll cut the time limit close every time. Used to be trioable with mages, but they adjusted it so that BLMS don't murder the walls as quickly as they used to.
PFC - Lebros Supplies - difficult to win in the first place, and strategies typically involve multiple pairs of Powder Boots (read: goodbye gil). Not recommended, and in fact many people recommend skipping this one even for rank-up purposes.
SP - Troll Fugitives - winnable with any party that can kill trolls at a reasonable pace. Likely difficult with 3, but easy with 6.
LC - Evade and Escape - all that's needed is MGS skills, and RNG sub (even at level 1) for Wide Scan is highly recommended. Not really any easier with more than 3 people than it is with 3. Pretty spammable.
C - Siegemaster Assassination - see Troll Fugitives, except rather than "kill everything" it's "kill a specific one that you don't know in advance". There are also repops and the one you have to kill is never a repop, so you have to pay attention. For all that, it's fairly spammable, but the LC Assault is probably easier.
S - Apkallu Breeding - use the map on wiki, have a sense of direction, and this Assault is so pathetically easy, spammable, *and* profitable for points that there's little need to do any other Lebros Assault (unless you're gunning for a rare ??? identification).
Have yet to try SM/CS.
Periqia:
PSC - Seagull Grounded - if you can figure out how to reliably control the NPC (it's difficult), this one isn't *too* bad, but it's still a little obnoxious. Technically spammable but I'd wait for a higher rank.
PFC - Requiem - mass slaughter of undead, no real complications, but you'll probably want more people rather than less.
SP - Saving Private Ryaaf - easily spammable even with 3, just need MGS skills for fomor-dodging, and chigoe-killing tricks like jumps or meditate -> WS.
LC - Shooting Down the Baron - only hard part, really, is tracking down the imp to kill (and the time limit). Otherwise not hard, probably trioable, easy with more.
C - Building Bridges - spammability purely dependent on MGS skills. No fighting, so 3 people are as good as 6, really. Note that MGS'ing in this Assault is harder than in some of the other MGS-related Assaults (such as Saving Private Ryaaf or Evade and Escape).
S - Stop the Bloodshed - finding the chigoes is difficult, and the true sight lamias are murderous *and* time wasters. Theoretically spammable but may be difficult to pull off consistently.
Have yet to try SM/CS.
Ilrusi Atoll:
PSC - Golden Salvage - spammable with a SMN provided that method still works. Doing it the hard way, BLMs own mimics and melees suck for them. The method for knowing a mimic in advance was removed.
PFC - Lamia No. 13 - spammable, and easy (even trio), provided you have dispellers.
SP - Extermination - another slaughterfest, so better suited for a large group than a trio.
LC - Demolition Duty - not hard at all, though you do have to kill stuff occasionally. Mostly it's just a matter of following a puppet around.
C - Searat Salvation - not too bad, just as long as you can avoid letting qiqirn NPCs and orobon NPCs near each other (which is tricky). Spammable with practice. Points dependent on how many qiqirn you successfully retrieve.
S - Apkallu Seizure - the apkallus are very fussy about when and how they'll follow (or, more accurately, lead), so it can be annoying, but would be spammable with practice. Points dependent on how many apkallu you successfully retrieve.
Have yet to try SM/CS.
Anyway, my advice on spammable Assaults from each, to summarize:
Leujaoam Sanctum: Shanarha Grass Conservation, Counting Sheep (EDIT: neither of these is *really* good, so maybe S/SM/CS has a good spammable one)
Mamool Ja Training Grounds: Preemptive Strike, Double Agent
Lebros Cavern: Evade and Escape, Apkallu Breeding
Periqia: Saving Private Ryaaf, Building Bridges
Ilrusi Atoll: Lamia No. 13, Demolition Duty
