I only casually speedrun GE(ie I suck at it) but there's still some levels I haven't even bothered with and probably never will due to boringness: (if I don't list what difficulty then it applies to the level in general, not any particular difficulty)
Streets- 100% pure running, or using a grenade launcher if you know where to get it(I don't) either way, still boring as hell.
Statue- all about how good you can run, and knowing how to do all the various tricks and not screw them up, and of course get a lucky ending. I got the cheat for SA/00A(forgot) and never touched it again.
Cradle- Again there's pretty much no variety here, you shoot 5 bullets infront of you, run, and either hope all the objectives get completed by a grenade or do the psycho-style headshot-console-headshot, which someone like me couldn't pull off so all I could do for speed is the boring grenade trick.
Runway A/SA- nothing to this execpt knowing how to throw the grenades and throw them right, and of course hit that damn battery that I always somehow miss unless I use a timed mine.
the rest, for the most part are bearable.