Jack the floor to level.

Thanks, Joseph. Ideally we’d level the floor, but we’re in a condo with a cement subfloor. It also wouldn’t fix the problem of the window frame being off level (it’s parallel with floor, which makes me think whole building has shifted over the years). The uneven distance from the level cabinet rail to the in-level window is very noticeable.

Remount the window please. It ain't all that.

IMO 1/2 “ over 10 ‘ is not huge and in a condo moving much is not allowed I think make the cabinets level adjust as needed to bring to level the trim above the cabinets IMO best to be as thin as possible and plain , I mean no crown molding and the tile taken to the bottom of the cabinets and adjusted as the tile setter goes along so that the tiles are adjusted where they are the least visible under the cabinets and under the window I think the adjustment for the tile will be minimal and many times walls are not level or flat and a good tiler knows how to make adjustments . I think I would talk to the tile setter first before choosing tile to make sure the choice is the proper one.

A picture of the window and the planned tile would help.

Basically you try to let everything absorb a little of the difference. The window stool, the window apron, and the grout lines. The general target is that no one item should absorb more than 1/32" per running foot, but the closer to other horizontal lines the more noticeable, so you might have to adjust that.

You could avoid tile altogether and continue the countertop to the backsplash.

So far all we have hung are the uppers and have staged the suspension rail for the base cabinets. It’s an Ikea cabinet system. The uppers are level and aligned. The suspension rail in this picture is also level. It’s hard to see in the picture but from one end of the window to the other there’s a 1/2in difference in distance to the line of the suspension rail. There’s still only 1/2-5/8in slope to the floor over 10 foot run, so window must be more un-level than floor. We’re considering getting a new window - with the way it is constructed, can a new window even be put in to be level? It looks like the drywall framing is square but not level. Second picture shows what we’ve done to account for not level ceiling.

Install the cabinet, then look at the window trim and see how bad it looks.

If it still looks bad to you, pull the apron, and then shim the stool to look straight. Caulk it back in, and tile. Of course this is going make the reveal on the bottom of the window and the stool look off, but it's going to be off some where.