What Pokemon to go for as guaranteed lucky?

In general I only consider luckies useful when there's plenty of candy available to me, and even better if I trade for legacy moves. I used my chances for a Confusion Eggy and I'd love a lucky Machamp, BB shiny Charizard or shiny Gyarados, for example.

BB Charizard, FP Venusaur, and SD Ttar

This is a quite personal choice. It depends on what you need. How does your roster looks like? Do you have 4-6 machamp, 3 dark tyranitar or SD tyranitar, a SD Mewtwo?
These and several other legendaries or future generations 4 evolutions would be my choice if I didn't have them.

Ask yourself what mons you want to max out, and go for those. I swapped Shadow Claw Gengars and also traded for a beldum in anticipation of community day. Rhydon and Piloswine aren’t bad choices either when gen 4 comes.

I would go for meta-relevant legendaries or shiny meta-relevant CD exclusives. If you want the maximum profit, you want to get a mon that you wouldn't mind powering up anyway and one that's hard to get the lucky background on otherwise. So Rayquaza, Mewtwo, Kyogre, Groudon, SD Tyranitar etc.

I agree with the notion that for a lucky pokemon to be useful, candy needs to be plentiful. As such, the most meta relevant non-legendaries would be my choice. Ttar (B/C or SD/SE), Machamp, etc. But of course, any of these could be upstaged. If Rhyperior gets a double rock move, he'll probably snag the rock throne from Ttar. Mamoswine and Magnezone could be comparable to Raikou and Zapdos. So choose carefully, it would suck to get your powered up lucky to become second best. Maybe wait for G4 to drop before deciding - we'll know a lot more about which pokemon come out good and bad from that.

I was in the same boat as you. I spent my last 4 guaranteed lucky trades on Mewtwo. 3 Shadow balls and one current one. The best thing is people are generally okay with trading a junk Mewtwo. And before shadow ball was gone people happily traded shadow ball Mewtwo. All ended up with useful lucky IV. And the non shadow ball one turned out to be a 15/15/14.

It's a lot of Mewtwo candy, but I'm good on anything else. I do a lot of raids so now I have a dump for all my rare candy.

In ract, for me, lucky Mewtwo was an easy choice.

Remember too, you have to have less than 10 lucky Pokemon for the older (8/16 or 9/16) Pokemon to guarantee a lucky. If you have 10 lucky already then the guarantee is no longer valid if I remember right. Lastly, don't wait too long....you may get lucky pokemon the normal way (RNG) which counts against your 10.

A similar thread from a couple of weeks ago that may provide you some useful input ;)
https://pokemongo.gamepress.gg/q-a/which-7-pokemon-do-you-want-lucky-if-you-could-choose-all-generations