I find that it helps my team tremendously to play a tank (solo queue), since no one else wants to play one.The problem becomes that when I have bad teammates, I can't do anything by myself.

So I'm wondering if I can get some suggestions of a tank that is flexible enough to do something else if need be. Rammus seems like an OK choice, since he can focus on AP + armor and use damage reflection to off some lone carry (useless in team fight though). Alistair can attempt to single out an enemy, but takes tremendous risk doing it (Running into a group of enemy when your teammate isn't helping = certain death, even if my team does get 1 kill). Singe has poison that could assist my team a bit more, and Kayle can turn into a DPS as soon as my team starts getting killed through Holy Fervor (but the problem is that we can't get a kill to begin with).
Am I missing anyone? Who's the best clutch tank out there?

My personal favorite is Dr. Mundo, he's a great DPS even though he's considered a tank.

Malphite is a pretty good semi-dps for similar reasons as rammus, his skills are good with armor. He also has brutal strikes which can increase his dps output quite a bit.

Sion and Nasus. Although these aren't often built as straight tanks, Nasus has Siphoning Strike which gives a huge one-off boost to his autoattack, plus having a slow, natural lifesteal and an AoE spell. For his part, Sion has Enrage, which not only gives an insane damage boost (+65 damage at lvl 5), if you get a last hit, it permanently increases your base health. You've also got a shield that can double up as a damage tool, a stun, and a lifesteal-based ult.

In a slightly different fashion, there's Cho'gath. His autoattacks, are a lot nastier than they seem thanks to Vorpal Spikes, he has a (rather hit and miss) AoE slow/damage, a silence, and a pretty nasty nuke. Plus there's the whole growing thing. It's entirely possible to hit 3k health by level 11 with him. However, he's a bit rubbish against towers, and Rupture is, as said, a rather uncertain tool.

If you're looking for a "TANK" with DPS, your best bet would be either malphite or cho'goth. I personallly suggest cho, the alien.

It might take some time to get used to him though. Take a few games, learn how to play him. He has both a stun and a silence, and can finish off enemies pretty quickly later on [even tower diving to do so]. He also has the highest HP out of anyone because of his feast skill. Others can get higher, but still.

I don't like mundo because he only has a slow, and such. Rammus is also a good choice. Taunt is almost the same as a stun if used properly, so that makes rammus quite powerful.

Sion, on the other hand, is who I always play. If you go DPS sion, you're not a tank. Just a slightly higher HP player. If you go AP sion, then you're more of a tank, and u have a nice stun/aoe spell. However, I think AP sion sucks...

Kayle is another option. Granted, kayle shouldn't be used as your tank [tanking items pretty much suck on kayle], but kayle can save the life of others so well if used properly. And you can tower dive with your ult + ranged skill to finish off wounded people.

Hope this advice helps.

blitz can do a pretty decent dps. his overdrive (w) makes his attacks faster, and his powerfist (e) doubles his damage w/ a pretty short cd

Nasus always seemed like the best dps/tank combo to me, pop that ult and go to work.

If you build Nasus with a sheen/trinity at one point u will be hitting hard while being tanky. Singed can pump out alot of magic dmg while being tanky.

for a pure tank, i like alistar. after chalice and glacial shroud, the glacial shroud adds a lot of magic damage and awesome cc to boot.

but i think what you're looking for is a nunu. nunu has great jungling ability, decent cc, great chasing ability, high magic damage, decent farming, and does well with semi-tank items such as rod of ages.