Gothic 2 still holds up as one of the elite RPGs of all time. The first Gothic is much rougher around the edges though. It has a lot more bugs and the controls aren't quite as refined as they are in the sequel. Still, to any RPG fan, both games are a must-play. Truly fantastic stuff, especially the second (with or without the Night of the Raven expansion - vanilla holds up very well).
As for Gothic 3, it's a diamond in the rough. On release it was atrocious, however now there is a small plethora of patches/mods available that make it worthwhile. The community patch, quest pack and content mod, when all combined together, make for a very fun truly open-world action RPG experience. The only problem that wasn't fixed is the frame rate, but that's because of a poorly optimised engine. It'll never run crystal clear, but on a modern machine, it shouldn't cause too many hiccups.
Also, I'd recommend playing Risen 1 (on PC, naturally). It's a Gothic game in all but name. While Risen 2 and 3 were vastly different, the first game is completely on-par with Gothic 2 in almost every positive aspect. It even has a better combat system! Gothic 2 already has one of the finest melee combat systems in any RPG, so that's certainly saying something.
So yeah, Gothic 1, 2 and Risen 1 are all definite must-plays. Gothic 3, with patches/mods, is worth a look if you can stomach the occasional lag spike. Gothic 3 has quite possibly the best-designed game world of all time.
EDIT: Also, never pay any attention to anyone who says the combat is mashing buttons. They clearly have no idea what the fuck they're talking about. Gothic 1, 2 and Risen 1 all have timing-based combat systems that require you to dodge/parry and attack at opportune moments when your enemy is vulnerable. In Gothic 3, with the patches, it's more about watching your enemy and exploiting them when they make a mistake and leave their defenses open. Anyone who button mashes in any Gothic game will just get killed over and over and then have the audacity to blame the game for their own failings.