Singed is one of the most versatile tanks in the game. I main him, and my KD ratio is a bit above even, with my assist ratio being through the roof! Singed's can have numerous roles in a team: He can push like a madman, he can set up ganks and stop runners, he can help his teammates escape with the use of his abilities, and he if played right he can even be the decoy for your team to destroy theirs! Not to say you won't be any destroying yourself, but this singed build is mainly a support singed. Either way, be prepared to DO WORK.
There are some hard toss ups in summoner spells, but I find the cleanse & ghost combo works the best. Cleanse will get you out of tons of stick situations, as will cleanse. Early game if you're nearing death and are getting CC'd, cleanse followed immediately by ghost can ensure your safety. Late game, when you've perhaps pushed a wee bit to far you can ghost, and cleanse if necessary, yourself to safety. There have also been multiple times when I have been five man ganked, am running for my life with my poison trail on an baiting, then right as my team arrives I cleanse and ghost out of the entire enemy group, and let my team clean up.
While this combo is great for escaping, it's equally good at setting up kills. Cleanse will stop any stun, slow, or CC when you're chasing the low health enemy, and ghost will give you the necessary boost in speed to to land your flip or at least mega adhesive. With cleanse and ghost, you should never miss a flip or low health champ kill. Ever.
Why NOT to get flash.
One of the most argued spells to get if flash. Everyone has flash! Flash-null on spells is hax, you can go through small walls, and it'll help you catch the counter flash. BUT, it is not what singed needs. Singed has everything he needs in his abilities to stop people form chasing, and if ultimately required after poison trailing for so long and throwing down a mega adhesive, the cleanse ghost combo will get you to safety. Another argument nubs give me is "BuT FlaSH caN getz you2 teh enemy 4 fLip!" Yes master nub, that IS right, but the problem is when you flash, if you aren't close enough for your flip, there's a big chance that champs gone with his 12 health. Ghost gives you a SUSTAINED speed boost. Even if they flash from a ghost, the improved version with your boots will be enough to still catch them for the flip or mega adhesive. The key with ghost is it lasts, unlike flash. Flash is hit or miss in my experience, while ghost is prime for Singed.
A majority of your masteries should be put into the defensive branch, hitting the necessities along the way. Go for defensive things, don't bother getting dodge or any of the sort. The second part of your masteries go in the utility branch. Get increased health/life regen, improved ghost, increased experienced, and then last point in increased max mana. (Or wherever you want to put the bugger.)
As shown above, mainly just get some mana regen. Singed is a man hog early the way you're going to be using him, so investing in a little mana regen will put you a step ahead of the opposing team. The less you have to go back, the more you can farm, the higher level and more money.
Passive: I feel kind of needs to be reworked. You will probably go most of the game without noticing it, even though you do get some health from it.
Poison trail: Amazing. Use it to farm minions, run behind a dying ally so he can get away while the enemy champ eats poison, help poison anyone chasing you, etc. Just great, and with your rod of ages/blasting wand just gets better.
Mega Adhesive: One of his most underrated moves! Skillshot slow which helps you land your flip. LEAD IT. Lead your slow like you would any other skill shot, and one thing people don't know is if you throw it on a character, once the command has been executed as long as they were under the reticle of the move they will be slowed. Basically, you do not have to see the slow hit them for it to slow. Often it will land and not hit them because they kept running, but they will still be slowed because they were in the spot when you commited the command.
Flip: amazing gank move, can also be used to save your life or another life. More on this later.
Ult: I use his ult in three main situations:
1) Pushing a tower. Increased damage and attack speed. can usually bring down to 1/2 health lvl 6 one wave of minions.
2) Going into a team fight: need as much power and health regen as you can get in a teamfight. Pop it off and go in headfirst!
3) When I desperately need health regen: This should rarely occur.
The move sequence is pretty easy, but the first move switches. Usually I'm with a dps who will carry ignite or exhaust. If this is the case, get flip first. Get to bushes early, wait for them to check then you flip, and your dps exhausts while you pound on them for 1st blood. If they come with the minions and don't check bushes, the best part is you can ghost unexpectedly out of bushes and flip. Works either way.
If you're sans-dps, go for poison trail and play it safe.
always get mega adhesive third, then level up poison trail 1st, flip, ult when you can, and adhesive last.
I'm getting tired, I will finish this up soon.