Utility infusions: Let's talk
Infusions are not really worth it if you don’t do FotM, since the stats or bonus they give are not great for the price: as you said you can always do better with boosters and food.
If I really had to choose, I’ll go with MF. It allows you to either max out your MF, or to take a non-MF food if you’re facing difficult content (Fractals, WvW, etc.) and still have decent MF. Karma can be a good choice too if you’re into making legendaries.
If your only use for karma is obsidian shards then karma isn’t all that useful anymore. You can get a lot of it from the bandit chests in Silverwastes. They can also be bought with Bandit Crests. The infusion gets you 1 extra obsidian shard around every 37th event.
I’ve got MF on every character. It probably doesn’t help much, but none of them seem all that impressive anyway.
Experience might be handy once HoT hits, assuming that it will affect the growth rate of masteries. Not sure whether it will or not, but it would stand to reason that if you no longer level up and gain normal experience once you hit level 80 once you get HoT (and ascended items are ONLY useable at level 80) it would have to boost mastery experience gain or else it’d be completely useless.
I use Karma as the others are not as useful anymore. MF isn’t that great anymore since they added champ bags and put almost everything into containers which are not affected by MF. Gold was only really used for dungeons such as the CoF farm but that changed with that one update years ago. I think the other one is XP which really isn’t a concern for most people as we have skill scrolls.