I am playing in a campaign where it seems I will likely always be a level behind the party. For some story, we play every Sunday for 5 hours, but I have work through it so I only get around 3 hours in.

We used to play so that it was a milestone format and I would level up with the party. We have now moved to an XP format and only presence gets you the xp (I think to avoid some people missing sessions and coming back still strong).

My problem is not with the format, but trying to work being a consistent level or so behind. So my question mainly is, what class/subclass works best when you will always be a level behind the party and trying to still stay relevant.

It just doesn't matter

There is no 'best' here and the bounded accuracy of 5e really makes this fairly simple.

My group had a rotating-DM campaign that people could hop in and out of playing based on their availability. We all had different XP amounts and the levels spanned everything from a difference of 1 level to some who were 2 or 3 levels back or ahead.

And there really were no problems for any of us. I couldn't attend a lot of the sessions, so my bard was behind in XP and I was still able to contribute quite effectively (some, like my bard, might even say more effectively than the others.)

Don't worry about being a different level, just enjoy it.

There is one thing to possibly worry about

You may be fighting monsters who would normally be above your paygrade. Maybe. But you're in the group and have everyone's support. The monsters may hit harder than you expect, but there's always a mix of times when you're built to kill and others when you're not as effective regardless of your level.

Each class is better at higher level than at the lower. Doing the same thing as someone else in the group who is one level further than you will always put you on disadvantage.

Try another approach:

Find a niche that no one else occupies. If a party lacks a healer become one.

If there is no thief/rogue, become one

If no one has high charisma and social skills - you get it. ;)

I have played once a long campaign without a "social person". Every contact with the NPCs was a nightmare. When one of the characters died, its player made a bard. It didn't matter that he is three levels behind us. We had to rely on him anyway.
And on another occasion the sole fact that a character knew Draconic as the only member of the party, gave her plenty of relevance while contacting the dragons and exploring ancient dragon ruins.

Actually, one more thing: you may consider making a character with charisma and social skills for one more reason - some DMs allow to solve social conflicts basing on roleplaying. If this is your thing, you could work around lower stats this way.

AL Already Works Under This Format

If you're not aware, Adventurer's League already works on this general format. You are grouped by tier, and within a tier you can have a lot of variability group member levels. Levels 1-5 are the first tier, 5-10 second tier, 11-16 third tier, and 17-20 the fourth tier.

In general, this works fairly well. The most dramatic power difference within tiers probably occurs most in the first tier when a 1st level and 4th level character are compared. A single good hit can drop the former, while the latter usually has more staying power.

The reason for the tier groups stems from the power jumps that occur at those thresholds. So within your group, you will find that you continue to stay relevant despite being a level behind regardless of what you pick. The times when you may feel like you're falling behind will be when the group passes through a tier threshold, but you will catch up fairly quickly once you pass through a tier threshold.