this is the 238457th thread 'bout this topic, isn't it? seems like SPI as a trait is underestimated alot... people only like the egyptians for their UU it seems. still, you're screwed if you rely on UUs requiring strategic resources like horses (warcharriots) or iron (praets) - that's why i wouldn't consider the related leaders as top choices.
my personal number ones are
isabella/hatse/, both SPI as my personal #1 trait and another trait to secure land. all in all i prefer isabella, with her UB you can break free out of the most ridiculous locked-in situations - lvl3 cannons out of the gate, enemy dead. in my eyes maybe the best leader, versatile and flexible. hatse is a little bit better at securing land, but that's what you HAVE to do because it's not that easy to break free of locked-in-situations as it is with isabella. hatse, on the other hand, is great for WC rush if you have to. but, being dependend on the resource, i prefer lvl3 cannons overall.
SPI gives you the ability to abuse diplomacy, and diplomacy is the best way to stay out of trouble. switching between building civics, war civics, economy civics just as you need.
#2 asoka/ghandi/ramm/monty/MM/justinian, all of them great UUs and good UBs, still they're less flexible compared to hat/isa/jus
saladin not in this list cause ... he's rather weak
#3 HC, obvious choice. other freakin' strong leaders: elizabeth, pericles, frederick, ZY, darius, WvO, gilgamesh
to play against? all techers. boring to play against, too easy to take out most of the time. shaka is a pain in the ~~~, but it's alot more fun to challenge him.