Need advice

Mash's skills are relatively cheap to max and provide excellent return for the investment, imo you should max her first. Medea lily's aren't really all that worth it unless you intend to use her a lot. For your DPS, Drake will net you a lot of friend points and make farming really easy on you if you max her, so I'd say do that next.

I don't know whether your Lancelot is a saber or berserker. If saber, then his skills are definitely worth maxing, he has arguably the best crit skill set in the game. Ruler Martha's skills are also fairly cheap, and the christmas event lottery should reimburse all the skill gems you need for her, both of them are worth maxing too. Up to you, really.

That said, the above were all just suggestions and I'd say just go for the ones who you use the most, they will obviously be the most useful for you.

Of those servants your best bets are gonna be Mash, Ruler Martha, Drake, and Archuria. Mash has an incredibly useful skill set that is extremely inexpensive to level up in comparison to other servants; Martha is the same way, although she's more offensively oriented. Drake is far more expensive, but the returns are worth it in my opinion. Archuria should be your go-to DPS against saber bosses, and leveling at least her damage steroids will be a very good investment.

Lily is good, but her skills don't see enough benefit from being leveled to be worth it imo. Beowulf has high damage, and his skills support that very well, but because he his only defensive skill is guts, he's very likely to die before you can get any real use out of him.

As for Lancelot, if you mean Berserker Lancelot I don't think any of his skills are worth leveling except arguably his 3rd one. His first skill is just as good at level 1 as it is at 10, and his second skill is pretty bad at every level. I think his 3rd skill has the same issue as his 1st, but others may disagree. Saber Lancelot is an amazing servant, and all of his skills are worth leveling to 10, as they all work together to make him an impressively self-sufficient and effective boss killer.