I personally prefer Grasp on Trundle all the time, as it provides both sustain and damage while building defensively (plus he can proc it with Rabid Bite, which is much more reliable than Fervor stacks). If I ever have to solo lane him, I prefer Doran's Shield or Corrupting Potion. I'm not an aggressive laner, as he doesn't really need to be aggressive in order to scale into mid-late game.
Ravenous Hydra isn't worth it imo. It's too expensive to rush on a champion that scales better with HP and CDR. Best case scenario, it gives you a ton of sustain as long as there's something to auto-attack. Worst case scenario, you have no hp or resistances, on a champion that only has a late game if he's tanky. The less HP you have, the less effective you're going to be, which is why Titanic Hydra is the better Tiamat upgrade. He loses nothing from rushing it, since it's composed of everything he needs to scale into mid-late game.
His builds are flexible, but you always want some combination of HP/CDR/Resistances. He has good steroids, which let him deal good damage while building tanky, but he doesn't scale as a damage source, so building more than one pure offensive item on him is a waste of gold. It should always have either CDR or some defensive stat attached.
This item should always be in your build somewhere. It gives him everything he wants and accommodates his kit perfectly.
This is another really good item on him. I wouldn't say it's mandatory, but as an immobile melee he loses nothing from building it. All good stats he can make use of. Randuin's has a weird build path now, so this item is probably the safer option regardless.
One of his best damage items imo, as it gives him everything he wants. However, you don't want to prioritize it unless you're fed (it's expensive and doesn't help with waveclear). It's a great item to have later on in the game.
This is a great item for a support build. Trundle's mana costs are real, so picking this up early gives him some freedom with his Pillar usage. I wouldn't recommend building this at top lane however, as early HP is much more important up there.
I've never been much of a fan of this item, as his ult is all he needs to shred tanks/bruisers. However, if you want some lifesteal this item makes the most of your money.
is a waste of gold imo, as he doesn't need it to do his job. He's not a carry and he doesn't scale like one. You have to give up an HP item to build this, which means you'll have less effective sustain in teamfights.
EDIT:
This is probably one of the best armor items right now for bruisers and it makes your 1v1 potential even stronger than it already is. I kind of want it to be nerfed because I hate seeing champs like Warwick buy it to deal with me, especially since he has a lot of sustain himself. It basically comes down to whoever builds this first wins, which is silly.
.
.
.
If he's being played by anything more than a complete potato you're not winning this lane alone. Rush Tiamat, build MR, farm passively/wait for jungler, outscale.
You cannot beat her by yourself. Rush Sunfire/Tabi (or Tiamat into Deadman's Plate) and do not give her a kill. Treat her like a ranged Pantheon, play safe and outscale.
You can beat him if he's dumb enough to get in your face, but don't count on it. He gets free resistances, which gives you an edge once you hit 6. A good Jayce is hard to deal with no matter what, but fortunately his late game is highly skill reliant, and as long as he doesn't get fed he won't be that big of a deal mid-game.
is a much trickier matchup. As you can't kill him if he doesn't want you to, you can't stop him from farming, plus he outscales you in teamfights. Don't pick Trundle into him unless you're sure you can put him behind somehow.
Bad ones are pretty easy to deal with. Good ones will kite you with Q and punish you if you go in too far and take minion aggro. You lose nothing from just playing safe and farming, as you outscale him hard (you pretty much counter him just by existing). As long as you don't give him early kills, his late game is pretty much ruined as long as you ult/Q at the right times.
Don't pick him into her unless she's being played by a potato, or better yet go ahead. Some things are best learned the hard way.
I'm tired of typing now. Basically, just don't build Trundle glass cannon, throw some pillars around and you'll be alright.