The best weapons in the game are all unique items. Among those, my favorites are Chrysamere (a two-handed long blade), Goldbrand (a one-handed long blade), Hopesfire (another one-handed long blade), and Skull Crusher (a one-handed blunt weapon). The best non-unique weapons in the game are daedric weapons, though there are a couple of stahlrim weapons that are just about as good as daedric. If you want pure killing power, you can make a weapon that is more potent than any of the unique items in the game by enchanting a daedric katana or daedric dai-katana with the most powerful drain health or absorb health effect that it can hold.
What armor is best for you depends on your armor skills. Different kinds of armor differ in quality, and high-quality armor will give you more protection than low-quality armor, but your armor rating for any given type of armor is also very much dependent on your skill with the class of armor (light, medium, heavy) to which it belongs. For example, if you are very skilled in light armor, and relatively unskilled in heavy armor, then a suit of high quality light armor could give you more protection than a suit of high quality heavy armor, even if the heavy armor is theoretically better.
The best type of light armor is glass armor. The best types of medium armor are royal guard armor, ice armor, and indoril armor (the armor worn by Ordinators). For heavy armor, the best stuff is daedric. There are a few unique armor items that are better than anything else you could enchant yourself. The Dragonbone Cuirass, the Ebony Mail, the Cuirass of the Savior’s Hide, Wraithguard, the Fists of Randagulf, the Helm of Oreyn Bearclaw, the Mask of Clavicus Vile, and the Boots of the Apostle are all examples of these, and there are probably a couple more I am missing.
As far as selling items worth more than 10,000 gold, you don’t really have a lot of options. If you are using Mudcrab or Creeper to sell expensive items, the best you can do is to sell them an expensive item and buy several less expensive items in the same transaction, to get the total price down to the gold they have. You then sell the less expensive items back to them piecemeal, as their gold regenerates, to get back the full value of the expensive item you started out with (you can wait for 24 hours, and their gold will have regenerated). For example, if you had an item worth 20,000, and Creeper had three Sixth House Bell Hammers worth 5,000 each, you could sell him the 20,000 gold piece item and buy the three Bell Hammers to get the total price down to 5,000 gold, which he can afford. Then you sell him the three Bell Hammers back, for 5,000 each, over the next three days.
Another trick for selling very expensive items is to sell them to an enchanter right after you enchant something. Getting a powerful constant effect enchantment on an item can cost you 80,000 gold or more, and immediately after you have paid for that enchantment, and until the enchanter's gold resets to their normal level 24 hours later, that enchanter will have all of that gold that you paid to them and can spend it on stuff they buy from you. So, for example, if you have a pile of daedric weapons and are going to enchant one of them, take all of them with you to the enchanter. Get one enchanted, and then, while the enchanter still has a whole bunch of gold, sell the other ones to them. You will not get as much for them as you would from Mudcrab or Creeper, but it is a way of selling a few expensive items very quickly.