Off-Topic: SpecsFlygon is actually pretty cool, and if you can believe it, it can get a "tad" bit better coverage than Salamence. Fire Blast / Draco Meteor / Giga Drain / Earth Power is usually the one I use. Specs Flygon also gets the ability to take on Swampert better.
Besides the point, here are probably the best "Mono-Teams" that I can think of, anyways.
Ground: Access to Swampert, Dugtrio, Garchomp, Flygon, Nidos, Gliscor, Claydol, and many others, including Hippowdon for Sandstorm abuse, Golem / Rhyperior can take advantage of the Sandstorm. Camerupt also shares Solid Rock like Rhyperior, just somewhat worse stats, and somewhat better typing. There's quite a bit of coverage from everyone for everyone.
Rock: Believe it or not, Rock has excellent coverage. As well as TTar starting Sandstorms, this makes Cradily a staple. Aerodactyl, Rampardos (to a lesser extent), Rhyperior, Golem, Solrock / Lunatone (TRICK ROOMZ!), Relicanth... the list goes on. Basically, anything with Aero, TTar, and Cradily, will cover some weaknesses that Rock has. Let's not forget Regirock, with his Superior 200 Base Defense + Sandstorm covers... a lot of things.
The reason to mention Rampardos is one thing: Mold Breaker.
Water: Thank GOD Water has some nice coverage. The mixed typing helps as well. Swampert, agian, is a mention, but Quagsire can absorb Water. Gyarados comes in the form of Intimidate, and Dragon Dancing, however, Feraligatr can brag some of this as well. Blastoise, Tentacruel, Starmie, Cloyster, and Omastar, come in the form of Rapid Spinning. Some have the Ice-Type as well, so it helps. Actually, Dewgong can come to mind with Rain Dance + Rest + Hydration. Ludicolo can benefit from Speed Boost from the Rain, or even some more welcomed HP for it. Speaking of which, Ludicolo is one of the only Waters that can SubSeed. Vaporeon can Baton Pass Aqua Rings... I guess. There's a lot of benefits in Water, no joke.
Steel: This is to a lesser extent, but comes in the form of Heatran. Skarmory and Bronzong can counter EQ, and if you want to consider it, Magnezone and Probopass can trap. Aggron is decent and cool and all, but needs help from surviving Close Combat somewhat. Scizor, Bronzong, and Forretress brag only being weak to Fire even, which Heatran covers. Steelix has Ground attribute, so he can take Electric Attacks quite well. And of course, let's not forget our sweeping friend Metagross.
Bug: Yes... Bug. Seems like a wierd mention, but it's true, Bugs are pretty useful. Forretress and Scizor return for some more fun here. Forretress is the Spinner of choice here. Yanmega can boast Speed Boost + Hypnosis, and can really put a world of hurt on teams. Shuckle comes useful here as well, with his High Defenses, expect him to stay a while. Heracross and Pinsir brag one thing: power. While Heracross benefits Guts, Pinsir brags Mold Breaker. Venemoth can take some hits like a pro, and Sleep Powder can come in handy somewhat. Plus, he suffers no Roost drawback like Scizor. Speaking of which, Bugs have a lot of Baton Passing, so this can come to mind as well.
There's a lot of examples of what each type has, but these, other than Psychic, which I didn't mention, because a lot of them have been mentioned in here. Grass lacks a little as far as some defenses, other than Toterra, but does come in the form of Status Effects. Dragon is pretty awesome and powerful, but Ice shuts it down. Normal has a lot of bragging power with Ursaring, Porygon-Z, Smeargle, and the ever-loved Togekiss, but also get Pokemon like... Regigigas. >_>
Just some examples. Steel looks interesting actually. Hehe...