I will be using it on more modern computers 2010+ , and a good gui would be nice too
Yes! I've been a pretty big fan of Manjaro for about 2 years now. Used it as my productivity OS up until about 5 months ago.
Knoppix. Unless you want it to have access to the Ubuntu store, then try one of the *buntus.
Edit: If you opt for Ubuntu, but are going to use it often on PCs without 3D accelerated graphics or with nVidia Maxwell GPUs, I recommend avoiding Ubuntu Gnome and regular Ubuntu. instead try Kubuntu or the lighter weight Xubuntu.
Linux Mint isn't bad, either.
I currently run mint on a computer, and it is nice, but is it lightweight enough
Lightweight enough for what? Mint doesn't do a lot of glitzy, clever stuff, so it's fairly middling on its resource demands. However, it's not as spartan as a console-only bare Debian installation.
Try both the Mate and Cinnamon flavors on a Live USB stick, and see what you think.
Thanks, I'll try both
Debian LXDE is quite a bit lighter than Ubuntu. LMDE and MX-15 are also good options for modest (but not ancient) specifications.
Thanks! Will consider
If you want to go with one of the *buntu operating systems instead, I would suggest the LTS releases specifically. The regular releases can be rather flaky. Xubuntu in particular strikes a good balance between features and performance from my experience.