Assuming I'll make it alive to lvl 14, my options from there on seem to be:
1. Mordenkainen Sword:
Rumor has it that it's THAC0 10, APR 1, and hits as +4 - if correct, I can't see what the hype is.
(e.g. Firkragg is supposed to be AC -12; so basically the Sword will maybe hit it every 10 rounds or so)
Its immunities sound a cheesy tanking option, which I hope SCS actually annuls. (I'm not sure, but SCS mages probably also know to Magic Missile or Death Spell them to death or just ignore the sword.)
In my tweak install they are "gated", i.e., they should not be susceptible to Death Spell. They are really annoying if hit by AoE spells of your party, if memory serves. (And they don't die easily: my recent Guarded Compound fight on the lower floor lasted some 20 minutes due to their Efreeti coming back like 4 times.)
Though I suppose Efreeti can be safely carpet-bombed with Incendiary Cloud/DBF/FB from the party and act as a perfect decoy. (Just not useful against Fire Giants and such...)
Djinn sounds like a finickier option, but perhaps offers other offensive capacity?
3. Pit Fiends [lvl 7 or 8 or 9]
Due to the competition of spell picks the lvl 7 version is perhaps best? Then there is the trouble of PfEvil management, in case someone uses Remove Magic on the party or you summon something else (Kithix, elemental staves etc.)
I imagine that Hakeshaar etc. are too risky and not worth the bother.
Skeleton Warriors may be nerfed in my install that replaces all +1 weapons with "fine" ones. (Haven't seen if that also applies to the lvl 15 version.)
Or should I just go summonless until I get to the planetar (if I survive that far).
I really would like a repeatable summon that can make a dent at the Dragons (which in my install have enhanced HP!).
Opinions very welcome.