One problem with playing online: Your character is deleted after 90 days if you don't use him. I've lost quite a few characters because I play games in "spurts", not playing them for months at a time while I'm busy playing something else.
Single player with the PlugY mod is my preferred mode now. It gives you infinite storage, infinite shared storage (so you can easily transfer items between all your characters), and lets you redistribute your skills and stats whenever you feel like it which is great for someone new to the game.
My favorite character so far is the skelemancer. I've also enjoyed a frozen orb/hydra and frozen orb/meteor sorceress (hydra more though). I even experimented with a sorceress that relied on casting a ridiculous amount of charged bolts.
For a skelemancer Clay Golem is useful, but don't put more than 1 point into it. I don't remember my exact skill distribution but I do know it was max Raise Skeleton, some Summon Resist, a lot in Skeletal Mages, and I think some in Corpse Explosion (which is by far the most epic skill in any game save for NWN's Epic Dragon Shape).
Like I said though, with PlugY you can experiment to your heart's content.