I think that the best vassal is Toku, because he is an isolationist.( This way I can give him every tech and not worry about him trading them.)
I think the worst vassal is Mansa Musa, because he is so friendly with everybody, and so willing to join your empire.

haha i'm the exact opposite on mansa. he's a good quick techer, so he's actually of some use. i can tell him research something i don't want to bother with if he has enough useful land left, or have him help me tech something, say liberalism, and then switch to something else before it comes in, so that i get to pick the tech ;). he's so very easy to get my relations back up to friendly with that he'll trades i put him on easily. i don't always enable the vassal option, but if it's on and he's nearby, i tend to start a war ASAP after one of us knows feudalism, cuz he really is nifty to have in my control.

so, favorite for me, Mansa. category you didn't ask, quite useful but requires the most effort and care in handling, isabella. worst i'm just not sure, the ones that i didn't like the first time i never take again :lol:.

i'm kind of like mansa as far as trades go myself. i don't worry too much about holding on to most monopolies. some i do (to try to reach liberalism first, physics, a tech advantage if there's a war imminent and the guy'll give it to my target). but if it's generic tech B7 and things are peaceful, i trade pretty freely feeling confident that i'll get more advantage out of it than that AI will. i keep WFYABTA in mind to some extent, but also have faith that i'll get somebody up to friendly up until it's their turn to die. that's clearly not your playstyle, so of course you don't want to hand mansa of all people techs like crazy, you want to horde your techs like toku does. part of what i love about the game, different strategies work!

the ones i didn't like:
cathy once, she was a pain in the candybutt
fred and peter, not worth it at all. didn't do a darn thing, just sat there.
louis, i didn't want track him down on the archi map but i should have to spare myself the trouble

i took isabella once, she ended up quite useful but in the middle of our relationship she was a ROYAL pain. she changed religions to one the rest of the world considered heathen. i couldn't get her to change to the proper faith, we didn't have liberalism yet for free religion, and of course everybody on the planet held her heathen ways against me too. naturally she was in theocracy, i had no way to change her out of that, so i had no way to push the right one to all her cities. she didn't have that many cities, seeing as how we'd had a war and all, so i made some new kind of junky cities near her, brought a missionary along with the settler, and gifted those cities to her. after a few of those she saw the religious light and converted on her own. it only worked because i'd had the brainstorm to convert them before giving them, so that theocracy wouldn't be an issue. after that, she was useful again, and the world did not hate her/me/us. but she can make diplomatic relations with others more trouble than they're worth, don't take her unless you're in the mood.

gus surprised me how good he was, but he was more stubborn than mansa about liking me again afterwards and trading with me. a little misunderstanding about a war, it took him a while to move on. julius was pretty good and a grudge holder as well, but gus was more useful. i'd take either one again if i did have the patience to finish them off.
mehmed was a good one. i'd take him again but the chance hasn't ever come up.
wang kon i took once and he was kinda good but he didn't enough land to help a lot. next time i'll leave him with more if i can, since it seemed he'd be a help rather than a hinderance.
i'd like to try liz or vicky but i never seem to get them in my warring games where i have random opponents. they're often in the games where i aim for diplo or cultural victories and hope for a world of permanent peace with no wars ever at all, and in those i take no vassals of course.

monty i've never taken since he just needs to die. i don't know if i've ever given him a chance to offer.
gandhi offers all the time, even during peace, all the time. one time, the turn immediately after refusing to speak to me since i'd cancelled trades with him due to somebody else's demand. i've never taken him. never accepted asoka either. hatty's another that offers in peace too often, never taken her.
never had toku, he's never offered that i remember.

how's that for a long answer to a short question? sorry *giggle*

mehmed was a good one. Thats good to know since I'm going to cap him in a couple turns!

Ghengis is kind of a fun vassal... he so loves to build military units!

if you want a military vassal, the Khans are very powerful... You just keep on feeding them obsolete units + military techs and they will do all of the warring for you - just make sure they don't free themselves!

The key to vassalizing an AI is to inflict more damage on it than you take (make it think they're losing), have at least one of double pop or double land, and have a military advantage overall. That and make sure you're not involved in some other silly war that makes them "afraid of your enemies". If you can vassalize an AI relatively early, you can turn them into your pet attack dog (yes, even Gandhi).

In my limited experience, the tech fiends make the best vassals, since you can outsource research to them and keep them from beating you to techs you want to reach first. Mansa, Gandhi, and Huayna are good vassals, the first two more so because they're also very forgiving.

I haven't tried vassalizing a military-oriented civ before in order to make them in to an attack dog; I'll have to try that sometime soon.