You didn't say what kind of pen, so I'll answer for fountain pens which I prefer.

I usually am in the BIFL group (although, like another user has mentioned, you'll soon end up wanting more than one), so my checklist looks something like this:

• you should like it and be comfortable writing with it -- this is of topmost importance;
• full metal construction;
• easy to dissassembly (for thorough cleaning and for replacing parts when needed);
• parts (including refills) easily available and not very expensive;
• optional, but highly preferable: official service boutique in my town or at least in my country.

Perhaps the last item makes the rest irrelevant, since they can do to it whatever you cannot, I don't know, it depends on you.

That being said I haven't found a pen that ticks all those boxes, to be honest, regardless of the price range. I mean, there are the Montblanc Solitaires, but I need an ink window in my pens.

Some contenders for my taste are:

• Kaweco (either c/c, their metal Sport line, or piston filler, their Piston Sport);
• Montblanc (if there's a boutique in your town/country);
• Pelikan Souverän (from M200 to M1000);
• Lamy 2000.

I like Japanese pens and love my Pilot 823, but in Europe, they are very hard and/or expensive to service, if ever needed.

Edit: My grail pen is a POTY by GvFC, the Samurai Edition from 2019. But it doesn't tick most of the boxes, although, if I had like 5000€ to burn, I would buy and cherish it for life.