Easily the most important thing are NP batteries. Reaching the 30%/50% threshold makes them like 3 times better than at 28%/46% because you can NP turn 1 with the right starting gauge CEs. This is straight up required for most farming setups. If you don’t care about 3turning I guess it doesn’t matter as much, but you should very much care about 3turning if you aim to ever max out the servants you care about, because you’ll spend ages farming what you need for that otherwise.
Second best would be any skills that have low cooldowns/high uptime already. Bringing skills like Charisma down to a 5 turn cooldown basically increases their uptime by 20% on top of the increased value of their buffs. This is a very noticeable damage increase during extended fights.
Third best would be the skills of your supports in general, since you tend to bring them to more fights than most of your other servants.
There are also specific skills that scale extremely well with level (Emiya’s Hawkeye) while others barely get better with levels, if at all and are thus not really a good investment unless you don’t have anything else to level first (all of Bedivere’s skills).
Stuff like Mana Burst is basically as good at lv9 as it is at lv10 since uptime is low to begin with and the buff value doesn’t increase much, so you can save your Lores for the more important skills if you’re short on them.
Yeah, Fionn is really good thanks to his multiple buffs. Him being bad is just a meme at this point. There are quite a few servants who go from mediocre-bad to really good post buff (Fionn, Astolfo, Emiya, Artoria, Kiara from the top of my head).