Which?

Pretty self explanatory what I'd be taking in offense/utility.

Which tree?

I actually take 9/0/21 or 0/9/21 depending on who I anticipate being up against top lane and if I feel I'll need the armor/mr at game start. Strength of spirit gives Singed borderline OP regen late game with all the mana you end up building.

I did go 1/8/21 singed before as mastery. But i did feel really week earlie game with it but it was really nice mid/late game. Now i run 1/21/8 and it goes much better. I now have the extra def that i needed at start when singed is not as his strongest.

i do 0-21-9, i take advantage of both swiftness and initiator. I bring mp reds ap per lvl yellows and blues flat ap quints. Rush RoA then i follow up with Rylais, after that w/e you need for tanky stuff depending on what your faceing FoN/warmogs/ran omen, most games my singed is unkillable 1v2-3 and does impressive dmg

I agree with above poster, move speed masteries are just too useful for Singed. I would run move speed quints > ap quints myself.

I use 1-21-8 for Singed. IMO, Singed requires ghost, and improving ghost is well worth it. I make sure to grab all the movement speed masteries as well. Singed has an extremely weak early game, not to mention all the times I've escaped with less than 20 health all throughout the game. That has cemented my decision about 21 defensive.

I can see 21 utility being useful as well, but the only reason to dig so far into utility for me is for the experience masteries. 1-3% spellvamp is a meh. Not useless, but not particularly more useful than just taking Vigor. Strength of Spirit is useful for him, but ROA is the only mana item you should be building in your core. Banshee's, Frozen Heart and Lichbane hugely situational, and you shouldn't be buying any other mana item on him.

EDIT: I suppose I should mention as well the runes that I typically use.
I do not use AP runes. Anywhere. Singed NEEDS his movespeed quints.

Here are two formats that I have used to great success:
Mpen Marks
Flat Armor Seals
Flat Mres Glyphs(Why not per level? Singed has a weak early game, and typically Merc Treads, FoN and Abyssal late game, still just my preference)
Movespeed Quints - At least 2(I prefer 3)
-----------------------------------------------------------
Flat Armor Marks
%Health Seals(My typical Singed build includes RoA, Rylai, and Sunfire, and Leviathan if I can finish boots and RoA in 11-13 minutes)
Flat Mres Glyphs
Movespeed Quints

1-21-8 here. I can't see anything in the first three tiers of Utility that would use that one point better than the ghost mastery.

Lol, I would completely disagree with you. I definitely build RoA, Frozen Heart, and Banshee's every game with Singed. His passive is actually very useful, compared to other champs. You should take advantage of his passive and make him even harder to kill. If you really wanna be "that guy", you could build Archangel's staff and get 350 health off of it and get mana regen and a bit of AP.

When I play Singed, my usual end-game build is Merc Treads, RoA, Frozen Heart, Banshee's, Force of Nature, and (insert any item you feel necessary).

I go Magic Pen marks, flat armor seals, and flat magic resist glyphs. I also get 3 Move speed quints. I go 1/21/8 and with no boots at lvl 1, you have 350 move speed; that's almost like having Boots of Speed on an AD champ, with his ult and ghost, you move faster than most champs can, even with Move Speed 2 boots when you don't have any.

Singed is definitely one of my favorites and it kills me when people say not to build mana items on him. *le gasp

I tend to leave Banshee's as my optional item (swapped out depending on how the game goes/what damage they have the most of), since just FoN, Merc's and MRes/Lvl blues seems to give me decent magic resist. I like going Rylai's for that slow on the poison, and the added AP is pretty nice too.