I will most likely end up making a Knight no matter how this turns out because stubbornness
Well as the main knight around these parts that's a good trait to have if you want to get to end game bosses. Now while i love the class i do have a hard time recommending it outside of a group setting. if you have people who are willing to carry you until you're useful, great, otherwise other chars can do better than knight at a lot of things while still being decent at what knight does well.
So pretty much every boss through dl is just "can you do enough damage" most don't even really care about a time limit and given enough time almost any char can beat these bosses. After that there's only a few bosses left to consider
rectus - can be alright, you can shield to take stalker explosions and give the ranged dps an easy time aiming.
Sb - pretty useless, sometimes you can protect someone a little bit longer.
ae- breaking is imporant, and knight does offer other protective capabilities that can help if your team lacks shielding from priest/wim. often times not as much as them, as using the ult to protect can often result in ent healing.
tl - with wof you can do decent damage on blues, otherwise you're just holding the main turtle in the middle, and several classes can do that
fn - you can kite easier and faster than any other classes (anyone who says otherwise is wrong, though this assumes roughly equal gear on all classes), a very important role for that boss. with enough gear and good kiting, a knight can solo fn, or have a reasonable duo with an actual dps (candies from upgraded jack bag help a ton)
sk - mostly you're just going to eat a stomp or try to hold aggro on the mini skeletons so the other people have less to worry about.
zl - actually really bad. knight taunts and has low dps which makes attacking the main body awkward, and taking one of the stars requires someone else to trigger it. so often you just run around stunning aco's during their coils, which many classes can do, several of which are better at it.
sam - mostly boxing. it's boring, but it's required to beat the boss and knight's natural ability to heal and live forever is pretty handy there.
sd - breaking and sometimes gathering drakes or protecting people with ult. and at this point there isn't really a better character for breaking as i believe lancer's cd is longer than 15s, but sd's a bit of a pain
if i missed any bosses, sorry it's 3 am, i don't really care that much to check, and this is a good sized list of endish game content.
as far as other tanks are concerned, lancer is somewhat tanky, imo not as much as knight, but it does have a break and some decent combo damage. There's the crusader, which again has decent combo damage, is quite tanky (i would say more than lancer) and while she's a great solo char (good damage, great self heal, some stuns) she just doesn't do anything well enough to really shine as a character. Dk and Berzerker and paladin can heal alot and thunderers tend to build items a bit more on the survival side of things, but i wouldn't call any of these real tanks, and knight's the only one who can really direct damage and protect others on their team. As far as items i try to keep them up to date in my guide along with alternatives and lower tier items, which are still good at the time with a snip-it about why they work well with knight which can be found here.http://www.twrpg.com/forums/viewtopic.php?f=4&t=21
for weapons you want winter's heart and oceanus or death scythe (these two do more damage than winter's heart).
for your helm it depends on the boss, if your group needs more protection the will of protector can provide more help there, otherwise halo of judgement helps with damage.
for your ring it really depends what flavor you like, bone is good stats and passive/active, ispion is great for healing people and otherwise living, but as dragon said it's supposed to have movespeed but currently doesnt and as many knight items don't have ms, until this does i can't really recommend it. i can see an argument for latea with it's absolute block.
end game armor is definitly creation in the next version it's active is going to include a group ams, which is very useful. otherwise earthen plates is good and so can be reactive armor if you need some more damage. sd's armor is also useful, though i personally don't care too much for it.
Wings can be tricky to nail on the head. yes grim heart is good, but frozen heart is average at best. wof is great for damage and has nice healing, whereas grim heart wont really be that amazing until late game bosses where your slight damage really drops off to nothing. sotd can also be good as you intend to take a lot of damage, and can die and deal some damage, as well as get back up and keep going. zl wings currently are nice because of their massive heal and movespeed in one convenient package, just don't bother trying to use the extra stats for much, as you don't have enough skill damage to make it work and even if you did, any reasonable dps char will outpace you in a heartbeat