Not Updated For Current Season
I was messing around with Singed, because I was bored and decided that I liked this build since it was very balanced having roughly equal AR and MR as well as a respectable damage output for a tank. After looking around I saw nothing really the same. Anyway.
I choose the flat armor seals and magic resist glyphs because they give a larger boost early game when he's most susceptible to enemies. the magic penetration marks because it gives a little more punch to his spells, but I could see replacing them with flat AP marks for more early game damage, or knowledge for a little extra Mana/Health, maybe just health per level marks.
Fairly straight forward building a Tank so you should max that and with Singed's passive Expanded Mind and Strength of Spirit of Pro choices.
Doran's shield to start over a Sapphire Crystal is not something i see much with Singeds, but with the recent buffs to the Dorans items I think it's worth it you'll have more health more armor and more health regeneration than with a Sapphire Crystal the only advantage for the crystal is that it builds into your Rod of Ages.
The boots of swiftness are for chasing down and flinging them back into your teammates. If you prefer you could go Boots of Mobility.
Rod of Ages is next and with Singed I this is a staple, Health, Mana, AP... Need I say more.
I build Force of nature next because I find magic and AP damage dealer to be the biggest pests in the early mid game plus the health regeneration is a huge to your durability and lane presence.
Next I go with the Frozen Heart for an armor and mana boost that adds health because of his passive as well as health regeneration (Strength of Spirit).
Banishes Veil adds everything mana, health, again boosts regeneration with masteries, and overall durability blocking spells (30s)
Spirit Visage means more health, regeneration, and magic resistance
*****If you're facing a all Physical damage team, however unlikely, get rod of ages, boots of swiftness, thorn mail, frozen heart, randuins omen, and a sunfire's cape.
Cleanse: removes CC because I don't include mercury treads
Exhaust: gas, fling, exhaust is great for early game kills
***** Only other spells I'd consider replacing is Ghost for Cleanse, but I'd change the build for Mercury Treads because CC reduction/ removal is essential on a tank (I think anyhow)
Pros / Cons
Regeneration is very Mundo-like
Fling is awesome when chasing/starting team fights
Gas good when being chased
Late game damage isn't overbearing
Requires good laning partner for early kills/harassing i.e. someone who doesn't run away when you fling back enemy